Freud's finding was that guilt is, starts off as an aggressive drive in the id that could go anywhere, preferably towards other people, but the superego uses some of this aggression and destructive energy arising in the id and then turns it back against the ego, and uses it to punish the ego, so the aggression, instead of going into someone else or into the outside world, is turned back against the self and to that extent is self-destructive.