Note: I don't have any links, the only way I can prove this to you is for you to play FFVI up to the point where you enter the Gestahl empire (not the magitek research facility). This is the part where you're trying to sneak into it, but don't go into the research facility!
Another Note: I found this out on the GBA version so I 'm not sure if there's a difference on the other versions.
Head to the northern most part you can go to in the village area of the Gestahl empire. I believe it's a pub or Inn. Talk to one of the people in there and he will tell you why Kefka is the way that he is.
*major spoilers for those that just want to know*
Kefka actually isn't crazy from his own will, nor is he naturally as powerful as everyone believed. He was the first magitek knight to be experimented on for enhanced abilities. However, since he was the first to be experimented on the process was unstable at the time. Due to the experiment being unstable it gave Kefka more power than he should've gotten and it severly damaged his mental status.
This will only take a moment of your time. *steals your watch*
