"Commodus is not a moral man!"

So why do you tell him he will be passed over for emperor when you die? In a room with nobody else? An immoral man might be inclined to kill you!

Of course Commodus murders his father having no idea who knows about his father's plans. For all he knows not only Maximus but his sister and every senator and officer knows about it. I guess he was just going to make a move for power and damn the consequences.