how come this doesn't work!?

You forgot to put '.' between Players and Veneus

wait(10)
game.Workspace.Players.Veneus:Destroy()

You forgot the period between Players and Venus.

The others are correct.

wait(10)
game.Workspace.PlayersVeneus:Destroy() --Incorrect

Is incorrect because you left out the "." which seperated Players from Veneus. The following code would work:

wait(10)
game.Workspace.Players.Veneus:Destroy() --Correct :)

Here's a tip though, let's say you want the player that you specify eariler in the script to be destroyed. You could say this.

Player = Veneus
wait(10)
game.Workspace.Players[Player]:Destroy() --Also correct.

The above example does exactly the same thing, just utilizing a new concept. You can see that the "." doesn't have to be used in this example because "Players" and "Player" are separated by brackets.