I had a similar experience, although my date did not turn out to be a mass shooter. He did later try to shoot someone. He was also a nice guy who was somewhat boring.
This was before online dating existed. He was someone I met in an English class. I only went out with him because I wanted to attend the event he invited me to and my steady boyfriend hadn’t asked me to go yet. I was tired of being taken for granted and so I decided to “show him” by dating this other person for this event.
Several months later another friend who had also had one date with him told me she’d seen in the news that he had tried to shoot his brother-in-law. Only then did I find out he’d been seeing a psychiatrist.
There was nothing out of the ordinary about our date. He behaved like a gentleman and this was in the 1960s. But something just seemed off.
Meanwhile, my mother, who was thrilled to see that I had dated someone else even once, was urging me to give him a second date. I admit that I shared the news report of the attempted shooting with great glee to her. She finally stopped bugging me to date someone else.