I don't think he murdered him just so he could be with the secretary, and firing him was not an option, remember that Stone made the comment, "you don't need anybody but me, and you can get rid of anybody but me cause I know where the bodies are buried Nelson I buried em for ya!"
No, the only way he could get Stone out of his life was to kill him, I guess he could not face a future with Stone always knowing his every move. Not that I am condoning it, but if he was party to murders in the past I can see why one more wouldn't bother him much.
sandy, i believe when harrry stone says.."you don't need anybody but me, and you can get rid of anybody but me cause I know where the bodies are buried Nelson I buried em for ya!"...that he is just saying that figuatively. it's not that nelson hayward actually committed other murders in the past. i believe harry was actually referring to the past discretions of hayward...lying, cheating, stepping on people to get ahead, and those sorts of things.
i agree, i asumed that he hadn't actually killed anyone but that harry knew his secrets from the past, suppose they could have been very damaging though...