I don't actually think he would. He wouldn't have enough company to make himself feel significant. I've seen enough "last man on earth" (or "last creature in place" if you want to correct me by being annoyingly technical) scenarios to know that this wouldn't actually be the best, and therefore most evil and selfish thing to do.
And I think he would still be. (here we go into ramble land, I hope you don't mind)
Discord in his prime, much like the Joker for example, will not commit some wrongdoing unless it is funny (or at least somewhat amusing) to him. So that limits him a lot because he is less interested about practicality. Flat out murder (if at all) without a superfluous set up and delivery would make him feel extremely uncomfortable. Discord gets satisfaction by making the lives of others miserable with irony and humor, and even then that's not even his main goal. He mostly just wants to have a good time, and is callous and arrogant towards other beings. In his world, he is the only person that matters. Which of course is why Fluttershy screwed him up so much with kindness and friendship. The two direct opposites of his malevolence.
Well obviously there is nothing humorous about Tirek's disposition. Tirek IS also all about himself being the one that matters, but with him it's more about dominance. Tirek's special ability alone tells us everything we need to know about his character. He takes power away from others to add it to his own. What he likes to do is brow beat all the time and be the boss. Like many villains, it's all about that power. What is power if there is no way to use it? Domination requires at least two parties, and to remove everyone else would ironically DISempower him.
I've actually seen some anime that dealt with characters who were able to copy the skills of others to the letter. While not of the magical-power stamina draining kind, the original masters of specific techniques were not flattered by the less than humble imitation. They then proceeded to teach said character a lesson by showing why not having the kind of mentality (very flexible, quick acting, able to improvise, etc.) needed to be able to naturally reach that skill level can become a liability in higher stakes situations (or at the least when the skill is genuinely tested).
I guess it could be like using a cheat code in a fighting game. The attacks of an opponent may be severely reduced, or simple punches and kicks can become instant K.O.'s. Still, that wouldn't change that, without such a cheat code, such a player simply wouldn't be able to hold ground against a skilled player.
Twisted as this sounds, all of the antagonists obsessed in conquering Equestria and/or the ponies seems to reinforce that Equestria and/or the ponies must have something quite valuable to attract such a passionate crowd in the first place.
I'm guessing when you have superpowers that strong, and literally just escaped from hell, thinking that far ahead may not come as easily.
That, or it just irks him that much that little girl toys (he probably does see the ponies as that) turned his brother against him and beat him senseless. Then again, the two did go to Equestria first as conquerors, so maybe they already decided to engage in a campaign (and Equestria just happened to be first).
If I had spent that horribly long in Tartarus, my plans about what to do after I escaped would be the only thing on my mind. Indeed he may want to crush all the little ponies, but that's not everyone. There's still all the changelings, griffons, dragons, cows, etc. that he could make into his slaves, all Sombra-like. I think you're right about how he feels about the ponies.
I suppose he could always sue for excessive force....
(pictures Tirek hiring lawyers crazy and/or desperate enough to take on his case, him genuinely demanding compensation in the form of mass magic absorption, and said lawyers actually supporting and fighting for this course of action
Now pictures said lawyers being changelings who also have an interest in absorbing the love from the ponies when the magic part is done)
Equestria seems to have some very weird and at the same time dangerous enemies.