The letters Mem, Vav and Tav are a sign for the Angel of Death
Synopsis
Rabbi Shimon works through some numerology and rearrangement of the letters in Mavet (Eng. 'death') to show that Pinchas was dedicated to the Holy Name of God and that he turned away God's wrath by wielding the spear. We are told that none of those who died in the plague were of Yisrael with the exception of those from the tribe of Shimon; those who died were the wicked, and the wicked are already considered to be dead.