Younger Than They Look: Danny Glover was only 40 years old when he filmed the first movie despite supposedly being "too old for this shit". Murtaugh was written as an older character (50 in the first movie).
Evil Counterpart: Like Riggs, he was an incredibly tough cop who bent the rules and got results, and they have many of the same snarky personality quirks. He's essentially a dark "what-if" version of who and what Riggs might've become if he'd continued down the self-destructive path he was on after Vicky died. Naturally, he and Riggs hate each other immediately.