First thought RUN, but you do make a good point about forgiving others and I do believe in second chances.

But I would brace yourself and sit down and ask him some tough questions.

What makes him a different person now than the one who was capable of killing his parents?

Is he getting ongoing treatment? (if not seriously RUN!)

With that in his record what are his realistic chances of finding/keeping a decent job and supporting himself? Feelings are great but relationships also have a practical side, you need to think about that too.

And I would ask your therapist to give you advice on other things to ask.

Also have him take off his shirt and pants (not to fool around) and have him explain the meaning of the tattoos he has, and why he got them, then do a bit of research online to see if what he says they mean is really what they mean. And yes people have to do some undesirable things to survive in prison, so his tattoos may not reflect his real feelings on things, but he will likely have a lot of explaining to do. If he is any sort of a person you might want to continue to see, he will understand that his revelation gave you a big shock, and that you need some time to think and process things, his reaction to that will tell you a lot as well. If he gets upset RUN, if he is understanding and willing to do what he can to alleviate your concerns then maybe, just maybe you could proceed slowly.