Good question, but unless you know why, it's silly to get upset about it.

Do you know the circumstances of his mother's death? Do you know his motives? Do you know of any extenuating factors? Do you know how old he was at the time? Do you know what he did while incarcerated to change his life?

Then of course there are the other factors. Civilised societies do their best not to lock people up for full life terms. Vengeance is a primitive emotion best kept out of the justice system. Different approaches to sentencing have different impacts on crime rates. The possibility of parole directly influences prisoner behaviour.

He was approximately 20 at the time btw, and stabbed his mother in front of his 5 year old sister, but while there was a previous complaint (when he was 18) of theft from his mother I can't find details of why he stabbed her in 2002.

Maybe he was trying to steal from her again. Maybe she was abusing his sister. Maybe she attacked him and he couldn't afford an attorney capable of demonstrating lawful self defence. I don't know, so I'm not going to get upset at the judge that sentenced him to '15 to life', or the parole board that released him only after rejecting him twice, or the city that thought taking someone out of prison and dumping them in a homeless shelter would assure avoidance of further criminality.

This is complicated shit with complex issues, strong emotions and very few 'right' answers.