I'll be very disappointed if they make him a Templar, even though they obviously will if he's in it.
He'd be a much more interesting character if he was just in the background, unaffilliated with any group, being the absolute terrifying lunatic he was in real life. I don't even want to see his face actually. Make it so you put a stop to the murders, but he still gets away without anyone ever knowing his identity. Make it so you only ever see him vaguely, and he gets away every time you come close.
Make him interesting and mysterious. The guy is like a folk tale come real, if they make him a known Templar, they most probably lose all of that intrigue.