I think the point people are making is that you don't always get a bad feeling about someone, that is true.
But that if you do get a bad feeling about someone, that is reason enough to end it. And she did.
Whether he's been previously reported or convicted is irrelevant to the OP's experience of him.
The OP stated that he has never had a long term relationship.
So I have a couple for thoughts about that.
Firstly, why? Have other women noticed this pattern of clingy and needy behaviour and ended it?
Secondly, are there no convictions because he hasn't been in a relationship for long enough to display any behaviours serious enough to report?
Of course, there is always the chance that he is a decent good guy, but only if you ignore the reasons that the OP made the enquiry in the first place, which weren't because she just wanted to be sure before she introduced him to her children but because he displayed behaviours which scared her enough to ask the police about him.