The tree's fruit

An evil tree
brings forth evil fruit.

It is always safe to eat the fruit of a good tree.
It is never safe to eat of the fruit of a poisonous tree.

A fountain that issues forth pure water is always safe to drink.
A fountain with bitter waters should never be used to quench one's thirst.

A good man out of the goodness of his heart brings forth good things in abundance.
They are beneficial for all.

An evil man is never to be trusted.