I swear I posted here o.o ~ Oh well, I can post again >:I
I don't have many insane characters, but the ones I do have don't just kill for the sake of killing, in fact, far from it. They always have some sort of motive behind their act whether it be something simple, or complicated.
One of my insane characters is Fallenspirit/star. She was fine until the murder of a close friend. It was after that she spiraled out of control and began to see his image everywhere... She was present while he was being torn open and had to watch as he flashed a smile and took his last breath. The sight itself was enough to drag her into a deep depression and then spit her out as insane. However, rather than killing every. Single. Cat in sight, she targets ones that look most like the murderer of her friend. She wants to avenge him and believes that any cat that even remotely resembles the killer should be removed from the face of the earth. Since she didn't catch the cat's name, she feels it better to rid all similar or identical cats so that she is 100% sure she's bought down the right cat and avenged her friend's death.
Still, with all this killing, there may be a chance it becomes more than just one here and there that looks like the criminal... It could become something she's passionate about, something that she feels gives her purpose.
Other than Fallen, my only other 'insane' character, is Alex. He doesn't exactly have a motive, he's just power hungry for all the wrong reasons. One day I'll figure out why, but until then he's just Alex the Rellik. xD