Madhu had received a weapon from Shiva, if he held it in his hands he could not be killed. He passed it on to his son, who soon became a plague. Rama and Lakshmana could have killed him, but Shatrugna asked for it, he also wanted to perform a heroic deed. Rama agreed.
Shatrugna learned that Lavana always went hunting in the morning and left Shiva’s weapon at home. He waylaid for him and killed him.