Basically you're at risk of getting killed in jail anyway, depending on what you did. Some of these guys go in as heroes if they killed people that were quote unquote "deserving", but stuff like mass murdering a marathon....nah. Some people do deserve to die for their crimes, though.
He killed two women in their 20s and an 8-year-old kid. There might well be prisoners who take issue with that.
I'm not sure that episode was advocating for that punishment.