I'm not short, I am just in travelling size!

Yes, Feanor was the best elf, and stealing and breaking the best of every group was Melkor's thing. Yes, he hated everyone in general and Feanor had a temper.

But I don't think that's all.

I think the main reason was jealousy for something Feanor had and Melkor didn't ever have and very, very much wanted to have since forever.

Feanor had a lot of unjust favoritism, positive bias and not always healthy love from his father. (Because Finwe felt guilty about the whole marriage situation)

Just imagine: you're a little narcissistic Vala, who thinks he's better than all your siblings (you remember when they didn't exist and it was much better), and that you deserve better, and should rule everything, but Iluvatar (the very name means "everyone's dad", not *everyone's dad but especially Melkor's") tells you that you're just as much valid as everyone else, and please stop messing up, and no, you can't be king of Arda, Manwe gets to be the kind of Arda.

And then there's this elf. Everybody praises him. His father lives him much, much more than the other children. He's the crown prince, the heir, the everything. And his personality is so much like yours, only he's more patient (which is of course unfair to you). And you warn him that his younger siblings will try and usurp him, after all that's what siblings do and maybe later he would help you get your siblings in their place. He threatens his brother with a sword and his father doesn't tell him that his brother is valid too, doesn't tell him to stop messing up, doesn't demote him. Instead, Finwe goes to exile with his son and it's very passive aggressive at everyone who was against Feanor.

And this throws Melkor into a rage and hating Feanor, because that's exactly the sort of behavior he would want from Iluvatar, but Iluvatar is, well, God, and not an emotionally immature elf traumatized by the death of his wife.

Seriously, very little things hurt as much as when you have an unhealthy expectation (especially after you've been repeatedly told it's unhealthy and started internalizing that), and someone else has the exact unhealthy thing happen to him and it works perfectly well for them. Repeatedly. I've been there and it's infuriating.

So add that to a divine power with full on narcissistic personality, and you have a recipe for disaster. And the disaster of course happens, with the trees and Finwe, and everything.

I'm glad Feanor died on the Balrogs, I'm afraid to think what would happen if he was captured alive (think: Maedhros but up to eleven).

And no, I don't blame it on Finwe only.

He was the emotional center, but he didn't do that much. Just some bad parenting. If he didn't remarry, of course... But also, if Miriel didn't say she won't be back (which turned out to be wrong anyway), if Feanor wasn't so jealous, if the Elves didn't go to Valinor in the first place (which was canonically a bad idea on part of the Valar, but also wouldn't be a thing if Melkor didn't mess Beleriand up. Of course it's still mostly Melkor's fault). So many ifs.

(I don't blame the Valar for letting Finwe remarry, I think he might do it without their consent too, and it's not really their business to let him or not. They might have given him some advice, but I doubt they had any good advice.)

So yes, Melkor bad, but the situation was triggering, and especially if you assume that he did reconsider his behavior a little during those 3 ages, some interesting what-ifs arise