The earl of Northumbria is rumored to have killed Ragnar by throwing him into a pit of snakes. Did many rulers maintain a pit of snakes for the purpose of execution?
It's usually suggested that this story (which dates only to the 12th century, about 350 years after the supposed events it describes) is a poetic invention - and even the historicity of Loðbrok himself is in some doubt. This earlier thread, featuring u/alriclofgar, goes into the problem in more detail. The thread also mentions other instances of snake pits being used as a method of execution.
As an aside, only one species of venomous snake - the adder - is indigenous to the British Isles, and it is scarce. While its bite can be fatal, it is not invariably so. So a pit full of adders would be hard to populate and not hugely efficient as a method of execution.
I think a better question is, did this actually happen?