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.
