True Believer Extremists and The Disaffected. Worth noting that disaffection often leads to true believer extremism.
True Believer Extremists - Politically motivated terrorists, foreign and domestic.
The Disaffected - those with severe depression and/or anxiety and/or PTSD with no prospects to improve their lot in life and choose to go out in a "blaze of glory".
Mental illness and drug use (including legal drugs, especially SSRIs) are catalysts.
The availability of guns is not a motivating factor at all, neither are violent movies and video games. That said, reducing the available firepower would save lives.