It would be fairly simple for me; I would have no right to turn him in myself by saying, "Yes, yes! He was here yesterday! He went thataway, get'm!". I'd just stay silent. I wouldn't have let him in my house in the first place, you see.
If he's caught and put on trial, then he's caught. I'm not going out of my way to turn him in OR hide him. If he asked if he could hide out in my apartment or something the answer would be a strong no, of course. However, I am not clean enough to judge other people for what they've done, no matter how heinous. He's already judged and damned himself a thousand times before the police could catch up to him, he doesn't need my hatred on top of that.
My answer to him? "I didn't see anything, I didn't hear anything. I haven't seen you today. I'm going to go inside, and play a video game. That's all I've done today. Hypothetically, if I had seen you, I'd tell you to decide whether you want to turn yourself in if you feel that guilty about it. I might also say that if you're going to be hiding, you should do so in a location that is not inside my house, but to do so quickly. Good luck, person I haven't seen."
Total potential crimes: Impeding a Police Investigation/Obstruction Of Justice