There's a line in Harry Potter and the Goblet of Fire; where Sirius says that you shouldn't judge someone by how they treat their equals, but their lessers. Along the same lines, I came up with this idea; don't judge someone by how they treat other people when said people are at their best, but rather when they are at their worst.
Let me draw that out a little. If someone is being nice to you, it's fairly easy to be nice to them. It's easy to be nice to your friends, particularly when they're being friendly. But what about if they're drunk? What about if they're asleep? What if they've been having drugs? Are throwing insults at you?
If, even after all of that, you can still treat them with kindness and some respect; then that is a far greater measure of a person than seeing if you can do the same while they're being nice to you.