I dated a guy once and his dad took me aside and said "you seem like a really nice girl...and let me tell ya....
"You DON'T want to get tangled up with my son"
He never explained why...but, for some reason...I listened to him. I mean...what dad would say that?
I only went out with him I think 2x and I don't recall any red flags...except what his dad said. After that...I was "too busy" whenever he called.
Found out later he beat the holy begeezus out of a gal that he dated after me.