After my BF died, I read his police report and the cop who put the report together got several things wrong. He wrote stuff that I had said and got it wrong, and when I called his friends about some things, he got that stuff wrong too.
It's possible they got it wrong about Facebook, but it would concern me, because a lot of people have been caught saying stupid shit on FB. I'd be concerned, because who admits to killing their sister? It's your sister, so WTF?
This is just me, but I'd pass. If it were me sitting across from him listening to it, I'd compare myself and my accomplishments to his, and I just think I'd pass on the fact that I don't want to invite his issues onto me, and you have already said he has admitted to issue. There is just too many fish in the sea to bother with someone who has that kind of background. I'm assuming he told you right away, because some other girl Google'd him and found it out the hard way. I'd also be too paranoid that my life would be in danger if we got into a bad argument or something like that.