Reference to an Aesop fable. In the original version, the scorpion stings the frog and both of them drown. The moral is that some people are intrinsically harmful, even against their own self interest
There's an old fable about the frog and the scorpion.
Paraphrasing, the scorpion asked the frog to be carried over the river, because otherwise he'd drown. The frog was reluctant, saying "but you'll sting me".
To which, the scorpion says, "but then I'd drown! so I won't do that".
Persuaded, the frog agrees. As they were crossing the scorpion stings the frog, and the frog asks, "Why would you do that? now we both die!"
and the scorpion replied, "because it's in my nature"