Sociopaths

The first and worst sign is sadism to animals as a child.
They have no remorse or guilt at all. No conscience.
Usually the sociopath will be a drifter with a history of trouble with the law.
They are shallow and exploitive in small things.
They are insensitive.

At an early age there is a strong bonding of aggressive and sexual drives in sadists.

When the military identifies these types they train them up to be assassins and snipers.