If Super-Duper Death Ray doesn’t resolve because its target leaves the battlefield or otherwise becomes an illegal target in response, it will have no effect. It won’t deal damage to any creature or player.

Super-Duper Death Ray targets only the creature. It can deal damage to a player with hexproof, for example.

If damage is already marked on the creature, that damage is considered when calculating excess damage. For example, say your opponent controls a 6/6 creature. Your first Super-Duper Death Ray deals 4 damage to it. Then your SECOND Super-Duper Death Ray (i.e. the one they never see coming) deals 2 damage to the creature and 2 damage to your opponent.

Trample on an instant or sorcery is similar to how it works on an attacking creature. If Super-Duper Death Ray would deal more damage that what it would take to destroy the creature (called “excess damage”), it deals the excess to the creature’s controller. For example, if you aim Super-Duper Death Ray at a 1/1 creature, it will deal 1 damage to the creature and 3 damage to its controller.