It's very likely you're referring to Wormtail, also known as Peter Pettigrew, who is a close friend of James Potter and Sirius Black.
Here's how Wormtail dies:
* He tries to kill Harry: In the final battle of Hogwarts, Wormtail attempts to kill Harry with the Killing Curse but is interrupted by Ron Weasley.
* His own silver hand betrays him: Wormtail's silver hand, which is made of Voldemort's soul, turns against him, strangling him to death. This happens due to the connection between Voldemort and his Horcruxes.
It's a tragic end for the once loyal friend who ultimately betrayed his friends and became a servant of Voldemort.