can they be excused

I don't think violence and abuse should ever be excused.

It depends on their ability to understand their crime at the time they committed it.

Just like a baby doesn't understand that hitting is wrong... some illnesses affect an adult in a way where they black out and have no knowledge of what they did. In this case, they can't be held criminally liable - but they can be forced to go to hospital and get whatever treatment they need to get better.

evil is evil.
you can beat it back. but you will never defeat it..

If they cannot understand, or were not aware at the time, they need help, not a prison sentence.

Ruining other peoples lives because of your problems is never excusable

IF the mental illness is known, and IF society fails to make sure the mentally ill criminals are committed and on meds, then there is an element of shame on society...but it is still no excuse.

Ha.. why are you pre planing some crime and wanna cry crazy to get by?

Personally i think the punishment should be implemented for any crime no matter how crazy the person is. They shouldn't be excused.