Honey, this is your first lesson in people can be jerks. A kissed you because he could, not because he wanted to. He didn't think it would affect you so much, and if he did, he didn't care. Let him go and move on.
If B is a nice guy, treat him as he deserves to be treated, and as you would want to be treated. Forget A, focus on B. He deserves it.
The day I met my hubby, a guy I'd just gotten together with had to leave the country. He disappeared for months and over time I started getting to know the man I have married. The day hubby and I got together, I found out the previous guy was on his way back home. I spent the next 3 days crying and stressing and not knowing what to do. I then realised that what I was doing was not fair on hubby. Needless to say I made the right decision as here it is, 8 yrs later and we're still very happy together.
I hope my story helps settle your mind.
'People are never perfect but love can be. People waste time looking for the perfect lover rather than creating the perfect love' - Princess Leigh-Cheri from Still Life With Woodpecker.