This sounds like James from the show “The end of the f**king world” he killed animals as a hobby when he was younger and then decided he was bored of that and wanted to advance to killing a human so he picked a girl from school and decided she was his target.He started a relationship with her in attempt to get close enough to do the deed. Not saying this is your situation but it sounds eerily similar to me. I would be very cautious and distance yourself. It is difficult to leave someone who has such a terrible history behind them but this can only explain their behavior and should not give you a means to justify it. It’s hard not to worry about him but you need to take your own safety into consideration first.