Those who enter through the back door can expect to be shown out through the window

Appearances are deceiving.

One who steals has no right to complain if he is robbed.

A wild boar was sharpening his tusks upon the trunk of a tree in the forest when a fox came by and asked, Why are you doing that, pray? The huntsmen are not out today and there are no other dangers at hand that I can see. True, my friend, replied the Boar, but the instant my life is in danger, I shall need to use my tusks. There will be no time to sharpen them then.

Beware of a wolf in sheep's clothing.

You can fool people some of the time, but you can't fool them all of the time.

A kindness is never wasted