The shooters are/were mentally ill-they either have no respect for life, no understanding of right/wrong, are hearing voices in their heads and/or are seeing hallucinations, have sociopath/ anti-social belief systems, or a combo of all of the above. These shooters are a class all unto themselves within the realms of insanity.
They are not committing random acts of violence to support a drug habit-they are mentally incapable of living any longer, as they have in complete horror and fear. However, I fear it is too late to have such a reasonable discussion that actually might go towards common ground solutions.