Gnam Esplosion

Roberto Manetti is the inventor of what is considered:
"The best chewing gum since Big Babol."
On his way home he comes across a man intent on fixing a bicycle.
He could stop to help him, but decides to go on his way.
A few seconds later he finds the man right in front of his front door.
What does the man with the bike really want?
This is the prelude to a long and contagious trail of blood