Have you ever had someone say, "watch out for that person" and then proceed to tell you all that's wrong with them? I have. It annoys me. It's gossip. It's petty. It could be sincere, but it's still annoying. I decided a long time ago to take people how I find them. I've had pastors warn me about someone coming to our church, but when they arrived I treated them decently, and didn't start out with the record already tainted. And surprisingly (or not) the person fit in just fine.
That's the problem with character assassination. Yes I called it that. Trying to paint people in a negative light, even when it's "justified" amounts to slander. Defamation. Character assassination. That's not to say that the person warning me was wrong. It doesn't mean that the person didn't do what they were reported to have done. It doesn't mean that I get along with people better than the other guy. It comes down to Christian ethics. How do we treat people according to Jesus? How we would like to be treated. (Obviously we aren't talking about thieves, murderers, deviants, and otherwise dangerous people.)
So when someone starts down the list of why you should "be careful" about that guy/gal, say something smart. Maybe this; "I appreciate you warning me, but I like to take people how I find them and make my own assessment." That usually foils the assassination plot. Remember, if you're a normal person you've probably done this. If you're just a regular guy/gal you've had this done to you. Recall how that felt, and refuse to be part of it. It never ends well.
Read the Bible in One Year Plan: Leviticus 26-27