We’ll see what how the trial goals. A judge would probably consider the risk of encouraging vigilante justice, ie letting individuals bypass the justice system to act as judge, jury, and executioner.

I’d be very surprised if a court excuse a vigilante killer because he/her suffer distress or harm. That would be a dangerous precedent, many people would see that as a right to kill for all kind of reasons.

It seems more plausible that such factor lead to that person to get a lighter sentence, rather than to receive complete pardon/mercy.