What stops illegal immigrants from murdering and doing other heinous crimes?

What stops civilians from doing the same thing? A armed citizen that can defend themselves. California is the worst because they have pretty much disarmed the citizens and when an illegal does commit a crime they lock them up at the expense of the California tax payer and instead of sending the illegal back when their time has been served they put them back on the streets of California. Its insane and California prisons are so full because of it they had to release something like 60 thousand prisoners early. You can see why California is in such financial trouble even though they have some of the wealthiest people living there.