I agree with Gray. Now is present, so you can't say something happened in the past now. You can say something happened "just now." That means only a moment ago. For example: "he died just now." Otherwise, you need to indicate that the action is complete by saying "he has now died." This means that now, in the present, the man has died, and it is finished. If he was still in hospital, and there was no way he would make it, then you might say "he is now dying in hospital." I would probably go with something simple like "He died recently in hospital." Also, let's not forget that we often use polite phrases for such unpleasant things as dying. For example, "he passed away recently in hospital," or "he has now passed." There's a couple of examples for you. I seriously hope you don't hit and kill anyone too often.

I agree with J. Ríos, and I would also add that "he died now" doesn't make sense -- "now" is the present, but "died" is past tense, so it doesn't make sense to say that someone "died now." However, you can say that someone HAS died now, because "has died" is present perfect tense, and it describes something that is true in the present (and is relevant to the present moment), although it happened in the past.

The man who I struck yesterday has died now is better to say because the action is recent.