He tackled him, barreling straight into his stomach.
The other was caught right off his feet. They both rolled on the floor.
One raised his head sooner. He reached for the blaster he'd dropped.
The first recovered in time and grabbed the other's head, slamming it to the floor.
He scrambled and straddled him, grabbing his shoulders to slam him backwards again.
Both grabbed at the other's face, covering their mouths, fingers seeking eyes, anything,
'til the one on top started to choke the one on the bottom.
That one's hands left his face to try and break the first's grip.
He couldn't. He scrambled, and found something on the floor. He grabbed it and smashed it over his assailant's head.
Again, they fell apart.
Again, one went for the blaster.
This time the man slammed his opponent with a chair. He bounced off the wall, where the man grabbed and again tried to wrestle him down, but they got turned around and the one being grappled slamed the man backward into the wall, himself. He broke the other's grip, jabbed an elbow to his face, and broke free, throwing whatever was nearest to hand backward to blind him.
Then they were just brawling, punch for punch, block for block, trying to get the other against the wall, until the one brutally struck his opponent down.
He knelt and again grabbed him by the throat.

"Don't you do it!" he shouted. "Don't you dare! Don't you leave her!"

A shot rang out. The man on the floor had gotten the blaster and fired it into the other's stomach.

But as his opponent fell away from him to the floor, Cassian let out a wail of disbelief and utmost despair.
Because his opponent was no longer who it had been all along: it was no longer
himself.

The body that fell dead to the floor, with a smoking blaster hole, was
Jyn.