Throwing out two other theories which are pure speculation and just my own opinion…
Maybe NM was responsible, paid by someone else to deal with Leah? Perhaps he had never committed murder before, only SA (not downplaying SA but there is a difference), and the murder affected him so badly that he took his own life? I’d be interested to look into his background and connections. Did he know anyone who also knew Leah?
Perhaps NM was indeed contracted to work at the property as a maintenance person via an estate agency rather than directly through the owner? This would mean others at the agency would have had access to the key, and that his suicide is just a coincidence? Or perhaps he made a discovery there and was blackmailed by someone else who had a key along the lines of “they’ll never believe you…with your record…they’ll think it is you so I’d keep your mouth shut”. Perhaps the knowledge ate him up?
Again, just pure speculation and MOO.