Premeditated implies some level of logical thinking. Mental illness means the individual is not thinking or has the ability to think in some logical fashion.
Evil - devil - these ideas mislead us from the cause and effect of mental illness.
Little social awareness of mental illness, in all its forms, has never been focused on and yet is is pervasive as cancers. My thought is this is because you and the President mislabel what caused this young man's behavior. Not guns or the devil.