Aunt May was shot by an assassin sent by the Kingpin to kill Spider-Man and his family. Mad with grief, Spider-Man captures the assassin who killed the man who shot Aunt May in order to prevent him talking, and shows him first hand what would happen to any other assassin who tries to hurt his family.
Trust Peter to use a science analogy to issue a threat.
From – Amazing Spider-man Vol. 1 #541
