to this:
Let the child understand that all people are different. If you inspire him that all good - then his first meeting bad uncle will easily take away as soon as you turn away. If you rub him that everything is bad - he will never have friends, he will forever suspect everyone and wait for a suspicion.
There are bad people and there are good people. Only actions can be determined.

by actions? Okay well. That guy with the candy is good, right?

Exactly by actions. A person of any gender who harasses others and offers free cheese is definitely bad. This should be explained not only to children, but also to adults. The more honest the eyes of the offering and more appetizing cheese, the harder you need to say "no" to the next fufflector steamer, a fanatic or an adept of an exotic sect.