If they're smart, or at least functional, plan out an attack against someone who specifically wronged them, and execute it specifically and well-planned to minimize externalities, it's a sign they're incurably evil and need life in prison. If they're just psychos who kill at random because they woke up and heard "Annie are you okay?" on the radio and then felt the need to kill everyone named "Annie", that's less evil, and so they get more lenient sentences.

It just makes intuitive sense, you see.