Locking gas caps only prevent the stupid from getting fuel.
As opposed to? If they were smart they wouldn't be stealing gas.
I'll state the obvious just one more time before I bow out of this thread:
If you employ deadly force to protect your gasoline, feel free to explain to the judge how expensive it is or how the thief limited your options in an emergency or whatever you like, but you will still spend the rest of your life in Federal prison.