List things that would make a decent looking man the least desirable in the world?

He's a serial killer or a war criminal. He has a serious drug problem or is abusive. There are myriad reasons why someone who looks good on the surface may not be worthy of someone else's time. 

Discovering your Yahoo questions.

Bad hygiene. Rude manners. Smoking. Dirty apt. or house. 

If he has an awful personality: like being arrogant, manipulative...

If he is decent looking, he can't be the least desirable.

I'm not going to be a big help but his overall vibe is very important. How he presents himself, the way he talks, the words he uses.