What made him so 'nice'?
I have a tendency to not trust seemingly nice guys outright. They seem too insecure to me in most cases, and insecurity is a dangerous thing to get involved with. It's this unfortunate dosage of insecurity that makes my own boyfriend both seem nice and an as*hole in one go.
It's complicated, but to be truly 'nice' would be a form of altruism that is very rare to find. There are various ways a guy can deal with deep issues of insecurity, and being overly accommodating, putting on an act of geniality, and/or ramping up innocent behaviour (sometimes to the point of appearing outright childish) are often coping mechanisms for people who want to gain acceptance. The flipside to this is that there is the dark side of an unstable ego, which can lead to the fickle behaviour you describe.
I advise you do your best to exercise judgment in these areas, whatever it takes. I personally try my absolute best to look beyond the superficiality of any possible facade a person may be donning, and the nicer, the more suspect, in my opinion. Remember, there is no such thing as true altruism, it's all just a complex interplay and all behaviours have their selfish basis. That's not say that compassion and empathy don't exist, in which case another's experience is seen as almost an extension of the empathizer, but you'll need to work on telling the difference.