Flying,
After it is all done with, they feel fear of being caught and often despicable for what they have done.
That is so with the common criminal. The serial killer who is more than usually a sociopathic killer has no fear at all. The feel nothing, fear nothing and can quite easily cook a pasta dinner over the bodies of their victims, turn the radio on and have a very nice time. They are born without conscience, that is why even as five year olds they can kill and dismember their family pets.
They do have a sense of euphoria during the long planning stages of a murder, during the murder and after the murder when they revisit the scene of their crime. Their senses are heightened to an almost supernatural degree during the killing stage, but then when this feeling fades, they must kill again to recapture the feeling.
We are all chemical creatures.
HS