why Hawkeye (Agent Barton) turned into Ronin(his new avatar introduced in Avengers: Endgame trailer)??

why Hawkeye (Agent Barton) turned into Ronin(his new avatar introduced in Avengers: Endgame trailer)??

Ronin

Hawkeye or Agent Clint Barton was introduced by Marvel in the very first THOR movie, But that was not a proper introduction. 
And Marvel properly introduced him the first Avengers movie.
Hawkeye is expert in Archery and martial art. If someone remember, We have introduced to his wife and childrens in 

Avengers: Age of Ultron movie.

Nick Fury helped him to keep this a secret from the world.

OK!, Enough intro about Hawkeye. Its time for the main juice:

Agent Natasha Romonof else known by Black Widow was talking about agent Barton to Bruce Banner, That he took retirement for spend time with his family.

It is possible that when everyone got vanish at the end of 

Avengers: Infinity War, Then his wife and childrens also got disappeared. But he didn't knew what was happening, Then probably he evolved himself with more brutality and violence, And got a new look and become Ronin.