So would you rather the property owner had shot and killed the trespasser as is allowed under the laws in some locales? (very rhetorical)
...unless you think the electrified sign is not dangerous or deadly.
Ah, the stipulation amongst carefully chosen words!
Dangerous? (able or likely to cause harm or injury) To a small degree it is. Many things in life are "dangerous". Do you still pick blackberries despite the danger of the thorns? (rhetorical)
Deadly? HA! Even small animals are not killed with such systems.
Well, that’s why we have courts, so individual controversies are solved - determining things like whether the particular electrified sign in question is dangerous enough to be considered per se illegal. Might be a close call, but generally I think courts frown on devices rigged to hurt someone especially if there is no warning sign.
Maybe the point is prudence would be on the side of putting up a warning sign. Otherwise it might be argued you wanted to see the guy trespass and attempt to steal so you could hurt him.