It's still their fault
Yeah, it's their fault, but they're kids. You can't throw a kid in grown-up jail for 25 to life. They will literally die in prison. Kids are in a special position in that they are
much more receptive to rehabilitation than even adults are (And rehabilitation has generally worked well for adults). They're also much, much more sensitive to punishment than adults are, and generally, if you can give someone a lesser punishment with the same result, you do it. To do otherwise would just be cruel. With special care and rehabilitation, and an appropriate level of punishment, they can get rid of violent tendencies and become regular people.