What happens if someone suffering from a mental illness commits a serious crime like rape or murder?

What would happen to someone if they were suffering from a mental illness and has been seeking treatment for it but then commits a crime against someone like rape, assault or even murder? im not going to lie and say its to help a friend, its to help me, ive been suffering from a mental illness for over a year now and every couple of months i struggle to keep control over it and quite often that results in unpredicted spells of anger or aggression, it seems to be getting worse and im really worried im going to hurt someone without meaning to. a lot of the doctors ive been seeing aren't being very helpful anymore and its very hard to keep working too. im trying to get better help but i want to know what would happen if i didn't do something if i lost control over it.