So I just read your story. And I have one of my own. My husband had a similar friend who was a bit twisted in high school. Really into horror movies and other things. Never got the full details on what he did obviously as he didn't spend 24 hours with the kid. But, a few years later when my husband was in his twenties he reads about a guy who drove his girlfriend to a hotel, drowned her in a toilet, stabbed her multiple times, and then shoved her dead body in between the mattresses and slept on her for a few days.

My advice? Don't be that girlfriend. Don't make excuses about how he really is nice just odd because you're going to be that girl in between the mattresses. If he's killing animals out of curiosity he's gonna get bored and move on to bigger prey. He needs help and no amount of kind words or adoration he shows you can change that. Good luck!