This is because when someone is going against your wishes, you would tend to refer to them as a derogatory term. To refer to someone with the word enemy carries with it a sense of gravity and respect. Therefore seeing a dead enemy has immense meaning which should be looked into more.
If your enemy is something like a concept such as capitalism or welfare or taxes and you see this enemy as dead then this is just a signature that your mind feels strongly about these issues. If you are not already, then you should become more politically involved. It seems to be what you really want whether you choose to acknowledge that or not, so this is the best course of action for you. You need to see what you can do and you need to see if you can make a change.
If you really want to slough off the chains of an oppressive regime, then do what you can to make sure that happens. If your enemy is just a regular person then a dream like this might mean that you really need to take a good look at what you are doing and make sure that you are on the right path. You don’t actually want to kill anyone. But if you are sure that everything you’re doing is safe then this dream just means that you will one day be able to conquer your foes and get ahead in life.
Seeing your enemy dead in your dreams is also a sign that the pathway is now open to achieving your goals. He or she who once stood in your way has been dealt with and now you are free to go on as you wish. You will succeed in life.