There is no excuse but there are reasons.
Remember Charles Whitman? The Texas Tower shooter? Had a brain tumor.
I agree but they try to turn those reasons into absolving the person of responsibility.
And as I said if they suffer from such a condition that can absolve them of legal responsibility for murder and violent behavior, then those people need to be locked up in a health facility and monitored and treated. Then they have a dangerous condition that prevents them from stopping themselves from harming/killing others and should be identified and treated for that, and kept apart from others so they can’t snap or break and murder innocent people.