Hawkeye becomes Ronin in the comics because of a combination of factors. There's personal tragedy, a sense of failure in his previous role, and a search for a way to make a greater impact in a darker world.
Well, Hawkeye turned into Ronin following a series of tragic circumstances. It involved the loss of his family and a sense of disillusionment with the conventional methods of fighting crime. This pushed him to take on the Ronin identity to take matters into his own hands.
Ronin is a masked identity that has been taken on by several characters in Marvel Comics. One notable character to use this alias is Clint Barton (Hawkeye).