Hippasus of Metapontum was born in the 5th century BC. It is believed that he proved the existence of irrational numbers at a time when the Pythagorean belief was that whole numbers and their ratios could describe anything that was geometric. Not only that, they didn't believe there was a need for any other numbers.
The Pythagoreans were a strict society and all discoveries that happened had to be directly credited to them, not the individual responsible for the discovery. The Pythagoreans were very secretive and did not want their discoveries to 'get out' so to speak. They all took oaths to ensure that their discoveries remained with the Pythagorean society. They considered whole numbers to be their rulers and that all quantities could be explained by whole numbers and their ratios. An event would happen that would change the very core of their beliefs. Along came Pythagorean Hippasus who discovered that the diagonal of a square whose side was one unit could not be expressed as a whole number or a ratio. Hence, the Pythagorean Theorem which crushed their original beliefs. Thus, they certainly didn't want Hippasus' discovery to be revealed and shatter their pride and core beliefs.
Unfortunately, very little is known about the Pythagoreans due to their very secretive ways. However, there are many stories about what happened to Hippasus as a result of his discovery. Some say that he was drowned, most believe that he was expelled from the Pythagorean Society and many believe he was killed and his discovery with stood the test of time.