Westcott has the strongest excuse. Even though his actions are the worst and FAR surpass his excuse, he was a kind and altruistic boy who he genuinely loved his mother and dog and was heartbroken when they both died. Then humans wiped out his entire race. Mysterio got fired and then became a mass murderer. Extremely petty especially seeing as how he was fired due to being unstable. Syndrome did genuinely want to be a hero but Mr Incredible saved his life so his excuse is petty to become a genocidal serial killer. Jafar was bullied for being poor and Drago lost his family and entire village to dragons. They don’t excuse their actions but they both have understandable reasons for why they hate the people of Shirbaid and dragons respectively.
Prob. Isacc. while it must suck to have your mom and dad die, it doesn't execuse mass genocide. Especially when your "freudian execuse" is bound to happen to everyone.
Mysterio obviously for me. Just because Stark fired him doesn't mean he has to go on destroy cities and his legacy, especially since in the last film, he sacrificed his life to save the universe, making the excuse even more petty