Spoiler Alert!
* During the Battle of the Department of Mysteries, Wormtail accidentally falls into his own Severing Charm.
* This charm was cast on him by Harry Potter during their fight in *Harry Potter and the Goblet of Fire*.
* The charm essentially forces his hand to sever itself, rendering him unable to use magic.
* He attempts to use his wand to reconnect his hand, but he's unable to due to the spell, and his hand bleeds to death.
So, while Wormtail doesn't die directly in *Harry Potter and the Deathly Hallows*, his death is set in motion during the events of *Harry Potter and the Order of the Phoenix*.