Because he’s their son. Because no matter what he did, they will always love him. Sometimes parents are able to cut off their kids, but it’s not an easy thing to do. That’s your kid. That’s who you spent 20+ years day in and day out raising and loving and learning. He’s facing his natural consequences by being in prison, and they have the willpower to not fight to get him out. They have immense love for the victim. But. He’s their son. Parent/child bonds aren’t always a thing (I can definitely attest to that- I don’t have good relationships there). But for those that do have that bond- nothing can stop it. There are interviews and such of parents of school shooters, and even serial killers. They all say the same thing, they hate what they did but they still love their child.