Am I obligated to...?

Last night someone asked me to help him. To me I like to help people in need. But about this guy, well I have to think twice about it. Why? Coz the way he treated me last time and even now. My other friends that know him, say that he kindda friend with a cause.. If he needs help then he will be nice to you.. Yeah at first my thought wasn't like that.. Then last few months, I noticed that it's true..
Well, he need a place to stay, which I have a room for rent.. Am I obligated to help him? I dont noe.. but what Karen C said to me, makes me think about it. What Karen C told me was right. Maybe I'm gonna let him stay in my house.. But it's gonna be different.. I'm not gonna treat him like before, maybe like a stranger..