The scorpion, in the course of a long journey, encounters a river. The scorpion cannot swim and so he is momentarily stymied until he spies the toad, on the opposite river bank. "Toad!" the scorpion calls out, "will you please let me ride on your back to the other side of the river because, you see, I cannot swim." "No way," replied the toad, "if I let you ride on my back you will sting me!" To which the scorpion replied, "But if I am riding on your back and I sting you then we will both die because I cannot swim." After thinking on it for a moment or two the toad agrees, swims over to the opposite bank of the river where the scorpion jumps onto his back and he turns to swim to the other side. When the toad, with the scorpion riding on his back, gets to the middle of the river, the scorpion stings him. "Scorpion!" Exclaimed the toad, "Why did you sting me? Now we both die."
"Because it is in my nature." Was the scorpion's response.
This is a folk tale, a parable. Basically, it stands for the proposition that we have to expect people to act in a way that is true to their nature. A person is not going to act in a way that is contrary to their basic nature, even if they fully intend to act differently in any particular circumstance. It's something to remember.