"We lost friends. We lost family. We lost a part of ourselves." These are the words spoken by the beacon of light, Captain America (Chris Evans), as he attempts to rally the troops in the wake of Thanos' successful snap. The fact that Joe and Anthony Russo lay those words over the first images of Hawkeye suggest that one surviving Avenger lost a lot in the snap, and it changed him.
Hawkeye (Jeremy Renner) will be Ronin in Avengers: Endgame, and he looks sick.
Ronin, in the comics, is a secret identity used by numerous characters, with Clint Barton being the second official person to adopt the moniker. It's largely used for a lone warrior, someone who is fighting as a samurai, and who needs to protect their identity because they plan to push the envelope. Comic creator Brian Michael Bendis initially planned for Daredevil to assume the mantle of Ronin, but plans shifted, and Hawkeye wore the costume in the aftermath of Civil War.
MCU fans have been predicting Clint Barton's eventual evolution into the role of Ronin, but something had to push him over the edge. Wanna get dark? Imagine this. The face that you see on Jeremy Renner above is the face that Clint Barton is making because he watched his entire family turn to dust following the Thanos snap. He's unsure what happened. But something inside of him broke.
Black Widow, knowing that the Avengers will need all of the help they can get, goes to look for Clint... who sat out the Infinity War. She's shocked at what she has found:
The tip off for the transformation is the lack of a bow and arrow. Clint Barton (Jeremy Renner ) has traded his trusty weapon for something more intense. A sword, which he cleans off on his arm, in a sleek, blood-wiping swoop. This man means business.
Black Widow and Hawkeye have a long history together in the MCU, as they talked in the first Avengers movie about missions they suffered through in the past. If anyone can talk Hawkeye back to existence, it's Natasha, though the assumed loss of his family -- that HAS to be what happened -- could mean that the vengeful hero is too far gone. Still, his need to avenge his family will have him signing up for the war on Thanos. But what can Ronin bring to that fight?