The Killer had to be eliminated as a loose end for his failure.
He would have been a loose end had he been successful as well.
There's equal motive to have him killed in either case
Speaking of which, the duo sent to kill him failed, but there didn't seem to be any repercussions for doing so.
Leo didn't need to be killed. He could have gone straight to The Boss to get information he needed.
I saw this a few months ago so maybe I'm not remembering well, but wasn't one of the guys who was sent to kill him the same guy who The Killer fought in the house where they were breaking stuff?
shareA living target is more motivated and has a better chance at providing law enforcement with some sense who the client might be and by extension revealing the contractors. So after a miss there is more reason to clean up loose ends.
When you fail to kill an assassin nobody cares because in this case the victim is unlikely to get the authorities involved.
However the play would have been another try at eliminating the target (but I guess the window had closed), if he was a productive assassin, makes no sense to eliminate him. Also very sloppy - and in this case costly - to get the woman involved. After their extensive stakeout the didn't even know he wasn't home??