Then you don't know what the difference between vigilantism and self-defense is.

One is a crime, the other is one of the most natural rights of life there are.

This sign is a warning against would-be intruders. Invasion of property is a direct threat, be prepared to face the consequences. Until then, we are good.

As long as there are no intruders, no criminals, then you don't have to take justice in your own hands. That makes the sign double-up as reducing the potential for vigilantism.