The Scorpion and the Frog fable says it all:
A scorpion asks a frog to carry it across a river. The frog hesitates, afraid of being stung, but the scorpion argues that if it did so, they would both drown. Considering this, the frog agrees, but midway across the river the scorpion does indeed sting the frog, dooming them both. When the frog asks the scorpion why, the scorpion replies that it was in its nature to do so.