Irenicus got mad for power and went way too far in his pursuit of it, but back then he wasn't a monster. He was simply too ambitious and not worried about consequences. So he had to be punished. To do so they took his soul away, and with that his ability to feel anything. Thus he became an induced sociopath who remembers what it was like to feel, but who cannot do it anymore. And he knows that he cannot feel, and he desperately want to feel, but he just cannot.
Thing is, the "more power at any cost" is probably the most overused motivation when it comes to villains. And the BG saga repeats it with Sarevok, JonBon and Mel. To me it is the hallmark of "safe writing", that is sticking to the tropes publishers know usually sell well.
This part is true. But the motivation is also somewhat relatable. Power is one of those concepts that universally cover all other things that we might covet, and being willing to break rules to attain something you covet is also somewhat relatable.
Jon, being a fantasy fiction villain, is of course taking that over the top with his power goal being godhood and the rules he's willing to break to get that being all of them. But isn't that mindset fairly common too? I seem to recall a former world leader who quite recently was willing to burn down his country to maintain his dream of being in power. I seem to recall the richest ever dude tweeting about a super-dirty crypto currency to enhance the value and make even more money. And so on, and so forth.
I think the bigger problem with Jon is that we never get access to his original thoughts before he messed up the first time. His hopes, dreams, fears, his plans for what came after, his moral conflicts if any, and how rationalization process to reach the conclusion that he had to go for it. And at some level, I think the silly "moah powah!!" motivation is even working for Jon, as it creates a contrast between the expected cartoon villain one would normally attribute that sort of simple motivation to with the obviously hyperintelligent but fundamentally pitiful creature that the punishment has turned him into.
Also, one might even argue that Jon isn't the only villain and maybe not even the real villain. He's a villain and certainly he needs to be put down, but he's also just a symptom of the complete disregard of others and lack of forethought by the Elves, who took a wielder of unfathomable magics, removed his ability to feel right and wrong, and then sent him out into the world to be a menace to everybody else.
