No matter how much they say they’ll change for you, they never completely do. While they may be treating you nicely now, be realistic and don’t expect it to be consistent or permanent. Some aspects of their true nature may involve asshole tendencies and, despite your good intentions to give them the benefit of the doubt, these never really go away; it’s the core of who they are. And when the honeymoon phase is over, bad habits resurface and will bite you in the ass, and not in the “oooh that’s hot” kind of way. Unless of course you like being treated like shit on a regular basis.
If you can sense they’re bad news, trust your gut. You’re probably right.