This is an in depth analysis of Tirek from generation 4 of MLP. I'll be going through some scenes and screenshots to explain why, at least in my opinion, Tirek is possibly the most interesting villain in Friendship is Magic.

First of all, Tireks opinion of Friendship, that it's worthless. After rewatching both Twilights Kingdom and Keep Calm and Flutter on, I realized Tirek may be on to something.

Going off on a tangent here, I think the reason Discord sided with Tirek is that he, too, saw Friendship as ultimately being far from the most powerful thing. Of course, Fluttershy had interest in being Discords friend, and this did teach him the magic of friendship...but the rest of the Mane 6 reacted with, at best, annoyance whenever Discord was around, and had no interest in befriending him. Like Tirek said, this was more like a prison, knowing you can be used for something and having a 'warden', so to speak, keeping you in check (Fluttershy).

Back to Tirek, his issues with friendship most likely stem from his 'betrayal' by his own brother, Scorpan. While Scorpan returned home, Tirek was imprisoned in Tartarus for a thousand years, fueling his contempt with the concept of friendship. After all, the magic of friendship CAN be turned into something else.

The next scene that caught my attention was when Tirek and Discord went to Canterlot. While Discord was free to have fun, Tirek went to get the Princesses magic, unaware that they had rid themselves of it. The scene caught me because of how forceful Tirek was. He knew what he was after, and wasted no time in crossing the room and taking it. If the Princesses had their magic or not, Tirek may have still had the upper hand in a physical fight. Also, notice the expression on Tireks face: It goes from angry, when he bursts into the room, to SMILING as he approaches his target. Not knowing about Princess Twilight Sparkle, Tirek has won and he knows it. Then notice how quickly it goes back to anger when he realizes the Princesses have mo magic. He was a second within winning and yet there's nothing more to take.
Then Tirek boasts, knowing that if the Princesses have no magic, he must have it all. He explains that everypony will bow to him, before locking the Princesses in Tartarus.

Now, this scene will take some explaining, so bear with me. Tirek and Discord are acting awfully...friendly, don't you think? This doesn't last long. First, Tirek and Discord have a little back and forth about who will rule.
DISCORD: You meant our will, didn't you?
TIREK: Of course.
Then Tirek gives Discord the medallion as they laugh about the new windows Discord made. Good times. Then Tirek snaps when he sees the image of princess Twilight. Discord has possibly betrayed him. There could be more magic out there, and Discord kept it to himself. Discord says he needed proof that Tirek considered them a team. Then Tirek GRABS him by the throat and demands to know where Twilights castle is, almost twitching with anger.
When Discord tells him about Twilights library, Tirek practically throws him to the ground, then instantly calms down and makes his way there.
Lets look at some of the emotions/expressions in this scene thoroughly.
When it seems Tirek has been betrayed again, he shows signs of Aggressive Anger, E.G yelling and physically threatening Discord.
Characteristics of Aggressive Anger are choosing for others ('why don't you have some fun', said by Tirek) and self-expression. Aggressive Anger also shows signs of distrust and fear, emotions that Tirek was justified in feeling, and this Anger leads to achieving your goals at the expense of others, just as Tirek did.

The next scene is where Discord betrays the Mane 6. Tirek drains the Mane 6 as we see Discord is having second thoughts. When Discord says that every 'Pegasus, Unicorn or Earth Pony will bow to us', Tirek immediately asks what Discord means by 'Us'. This exchange mirrors the last, as when Tirek talked about ponies bowing to his will, Discord asked if he meant both of them. Tirek, most likely lying, says a simple 'of course.'. Another characteristic of Aggressive Anger is being inappropriately honest and tactless.
Notice how Tirek only started outright displaying these features after Discord didn't tell him about Twilight. It's very likely that Tirek had deduced that Discord betrayed him, and decided to ensure it didn't happen again.
Now, an interesting thing to note is that Tireks behaviour almost directly mirrors his brothers. Scorpan most likely gave Tirek the medallion just before or after he 'Betrayed' him. When did Tirek give it to Discord? When Tirek takes Discords magic, this could be seen as crossing the moral event horizon, and by all acounts it is. Tirek had drained everyone but Twilight, betrayed Discord and imprisoned the Princesses in Tartarus.
Yet, there's one more scene for us to go through...

Tirek goes after Twilight, outwardly determined to take what he thinks is rightfully his, saying that it belongs to him. Tireks stance also shows aspects of Aggressive Anger, as widening of stance can be a non-verbal expression.
Now, ignoring the fact that Tirek almost outright tries to kill Twilight, lets move on the battle.
Between Twilight launching her first and second attacks, Tireks expression changes from anger to...almost tenacity. At this point, Tirek makes it clear that nothing is going to stop him achieving his goals, and as the smile as he fires his first attack shows, he's definitely NOT going down without a fight. The fact that after just one attack Tirek realizes what the Princesses have done with their magic, proves that Tirek is pretty deductive.
Now, through this entire fight, Tirek takes more physical measures in beating down Twilight, launching her into mountains and raising rocks out of the ground to slow her down. Notice that, before he throws her in the mountain, he has the perfect opportunity to drain Twilights magic. He did the same as he did with Discord, holding them by his magic, yet he chooses to continue attacking. While we can attribute this to the villain ball, think about this:
In the next scene, Tirek has captured Twilights friends, including Discord. Tirek says that he and Twilight are at an impasse, they are both equally strong. Assuming Tirek was wearing her down with his fighting style, he now has her friends and is perfectly able, and willing, of hurting or even killing them. Tirek is a master at manipulation and it shows. When Twilight agrees to give up her magic for her friends, Tirek drops them carelessly to the ground before painfully draining her of her magic (she cries out in pain as it happens). Tirek also displays confusion or knowledge when Twilight orders him to release Discord. Tirek can't understand that someone who was betrayed would forgive who backstabbed them.

Now, Tirek has won. He has practically all the magic in Equestria and there is nothing to stop him. When the Mane 6 get rainbow power, he still attacks and fails to recognise that friendship is a kind of magic, nor foes he realize that forgiveness, acceptance and second chances are also key elements in its use. Overall, Tirek was defeated not completely by the villain ball, but by his complete lack of knowledge when it comes to friendship, magic and all that it implies.

This all shows that, ultimately, Tirek is irredeemable as he cannot accept or forgive those that betray him, as his mindset will not allow him to. While we don't know how long Tirek has had this mindset, it most likely surfaced from his betrayal by Scorpan, and his centuries of imprisonment it resulted in.
Tirek is a villain who actually adapts to what happens around him, and can easily process information while keeping a somewhat unnerving personality, as evidenced by the amount of times, and how quickly, he goes from anger and violence, to determination and force and finally to victory and boasting. Though most strikingly, how he goes from using his newfound power in a victorious and clearly less sane state, to confusion and breaking down when that much power (like rainbow power) is used against him.
Tirek is also great at manipulating everything to work in his favour. He wins by picking up on the small details around him, offering a trade of Twilights friends (or their lives) in exchange for her magic, and playing Discord like a fiddle until their betrayal. Its these small moments that show Tirek had every means to win, as he ultimately did before his defeat.

In the end, Tirek is a very complex character who will do anything and everything to achieve his goals, regardless of who is in his way or who gets hurt. He goes from every emotion quickly and manipulates everything into his favour, and is not afraid of using force to get what he wants.

Ultimately, Tirek knows exactly what he's after, actively goes out to take it by any means necessary and gets it. While only for a while, Tirek wins, and it is only though his own lack of understanding and knowledge of the Magic of Friendship that he is defeated, making him possibly the only FIM villain with absolutely no hope of being redeemed.