If a man walked in on his wife in bed with his best friend and killed both of them in a fit of "passion," that man may be eligible for an insanity defense under the:

irresistible impulse test

This is a powerful drive for self protection as an excuse. The condition is often looked at as some kind of madness, in which the respondent contends that they ought not be held criminally at risk for their activities that violated the law, since they couldn't control those activities, regardless of whether they knew them to not be right. It was added to the M'Naghten law as a reason for exoneration in the mid twentieth century