Let's use our imagination for a little bit. Let's say you are getting mugged by a large man with a knife. You wrestle him to the ground, and as he falls, you grab the knife. He lunges for you at the last second and you stick the knife out and he lands on it. This is the instinct to protect yourself kicking in. True, the man dies, but it was self defense. If you attack someone and only wanted to hurt them, but kill them in the fit of rage, then there is consequences. You aren't evil, but have a bad temper and need some help.
But in terms of heat of the passion I think they deserve forgiveness. Chances are they were really insulted in a bad way (for instance someone talked about their girlfriend or mother or father in a bad way). Or in some cases, someone raped their daughter. In that case I can understand the heat of passion.
Because psychologically we know if sorrow, regret, and reality comes back after adrenaline drains out and Dopamine levels change that you have a technically "normal" human reaction. It's when the person gets some sort of kick/high out of the pain/suffering of another inflicted by their own with no remorse/regret.
Neurologically recently they scanned the mind of serial killers and violent criminals compared to a "normal" control brain and saw a large difference in frontal lobe.