View on "insanity" defenses

Alright, so you know when someone claims they are mentally unstable, schizophrenic, psycho etc. in court, all the crimes they did (no matter how heinous or violent) will get dismissed because the person is unable and unstable to stand trial.

Me:
Well, if you didn't do anything leading up to your age now, why did it just now happen? You grew up knowing the laws and the goods and bads, yet you act now. You decide to break the law and so you don't get in trouble (either because you snapped or you're a manipulator) you decide to plead insane. Grrrrr, ultimate trolls.

So, lets discuss some views and opinions on this!