A killing of a red snake in a dream can be because of an external stimulus reason.
For anyone, recently in actual life seen bloody scene of a killing of anything by anybody or a neutralization of a blood sucking mosquito can be a source for dreams. In order to kill a red snake in a dream is enough to see in waking life something that resembles such act.
For a woman, a finalization of a menstrual cycle can be an external stimulus for a dream about a killing of a red colored snake.
For a man, a release of blood from erected body part can be seen as a cause for the dreams about the killed red long limbless reptiles. And the relaxation of the organ can be felt up to few times during a night sleep.
Also, if you move year head and your closed eyes away from morning sunrise red light, then this removal of external stimulation can be used by your mind to recreate a dream in which you kill or someone else does it to the red color snakes.