Harris was apparently a full blown psychopath, and Dylan Klebold instead was just severely depressed. Klebold was more the follower type and Harris was the messianic type. Horrible crimes with accomplices typically have paradigms like this. One is the messianic leader and the other is the follower.
It is not often known, but depressives can be quite angry. I have known many depressives, particularly older females, in whom the main symptom was simply smoldering rage. Many depressed men are quite angry. Quite a few mass shooter cases which often end in suicide or “suicide by cop” are done by depressed males. Homicidal fantasies are quite common in depression. Not as common as suicidal fantasies, but still common. Look how many homicides are murder-suicides. These are usually done by depressed males going out in a blaze of glory determined to take someone with them.