Smart policemans

A homicide team enters a crime scene where a dead body of a fat old man lies there on the floor with blood oozing out of his head. The victim is holding a gun and a tape recorder lies there by his side. One of the detective picks up the recorder and plays it “I am tired of this life and hence I have decided to relieve myself from the worldly pains”. A gunshot follows the message.

The teams starts investigating a murder investigation.

Why?