Riddle of the Day!!!

It was the first day of college when a young girl was found dead on the college premises. The police narrowed down four suspects.
1) The Dean – He claims he was in his office the whole day.
2) The Maths Teacher – She claims that she was giving her students grades.
3) Peon – He claims that he was bringing the mails.
4)  Mathew РHe claims he was cleaning the roof of the college.

Who is the murderer?

Riddle of the day!!!

A homicide team enters a crime scene where a dead body of a fat old man lies on the floor with blood oozing out of his head. The victim is holding a gun and a tape recorder by his side. One of the detectives 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 team starts a murder investigation. Why?