Pythagoras was a cultist leader of the Pythagorean School in ancient Greece. He believed that the entire world was created by numbers and certain numbers had properties or personalities. For instance, 3, my favorite number, was the number that represented masculinity. To him, math was his religion and his explanation for the universe.
For some reason, Pythagoras also thought he was at least semi-divine and his followers had absolute faith and trust in him, despite the fact that this was the same man who thought he could write on the moon by writing on a mirror with blood and facing it towards the moon. (You have to wonder why he didn't just test that out and see that it didn't work...) Because of his followers' absolute dedication, everything that they discovered in math after Pythagoras' death was still attributed to him. So, we don't even know if he was the one to come up with the theorem that bears his name.
Without a doubt, the oddest legend regarding the Pythagorean School was a result of the controversy surrounding irrational numbers. Now, I know what you're thinking: irrational numbers... controversial? YES. The Greeks loved order and thought that everything could be represented simply. But when they tried to find the length of the hypotenuse of a right triangle with two sides that have a length of 1 each, they calculated to find that c was equal to the square root of 2, which could not be represented as a fraction or any other "nice" number. This bothered the Pythagoreans, to put it lightly. They ignored irrational numbers to the point where they were forbidden to talk about it outside of their community, but there's a story that one member of the Pythagorean cult wanted to share this information with the outside world. So the Pythagoreans drowned him.
Pythagoras lived so long ago that we're not really sure what's true and what's not. He was a mystical figure even while he was alive and that only increased after his death. But the important thing to remember is: if you've ever been embarrassed because you didn't understand your math homework, at least you didn't drown anyone over it.