Perhaps we just have a different idea of what "wrong" is. He failed in everything he did, yeah, but I don't think he was necessarily wrong in any of his judgements.
He had reason to believe that an invasion was immanent. Assassinating a king before an invasion to ensure a successful campaign is not a particularly surprising move. His attempt to usurp the throne, as well as the assassination of a boy king that talked to birds and was convinced to return a weapon to their enemies were both justified. And make no mistake, Zym is a weapon. If we're talking about some baby that may or may not grow up to be Alexander the Great, that's one thing, but Zym is absolutely going to grow up to be a dragon that can wipe many humans out single-handedly. Telling only his son to kill them makes sense because he couldn't trust anyone else, and there is no reason to believe that he hated elves any more than any other human. Even Harrow wasn't too upset at the idea of killing a couple would-be elf assassins. Aaravos introduced himself through dark magic, which Xadia hates, and offered real solutions to convince half a continent that they were going to be wiped out. And how is collecting people, especially assassins, in coins worse that collecting them in cells? Do you have any proof that he coined innocent or non-combative people?
No, I don't think he's the mustache-twirling villain you make him out to be.