Someone has the opportunity to kill you tomorrow, steal all you own, and get away with it. Forgetting conscience, there is no logical reason for them not to do it if you and they are only an animals. Animals kill members of their own species all the time for far less.
Its called survival of the fittest.
"Survival of the fittest" has nothing to do with individuals, it is an evolutionary concept concerning species and sub-groups and their fitness in the environment they live in.
Also, if I'm a born-again Christian, forgetting conscience, there is nothing to stop you from killing or stealing either since you are already guaranteed your place in heaven.
Anyway, most people can't afford the protection needed to prevent friends and relatives of the victims of their crimes from coming after them and exacting revenge. Forgetting conscience, there are still plenty of reasons obey society's laws. Anyone with half a brain knows it's beneficial to them to live in a stable, law-abiding society rather than a place where anarchy reigns supreme.