He felt that leaving his informant Tivik in-situ was too great a risk. The stormtroopers were on the lookout for suspicious types and he had no means of extracting him.
Tivik recognized his intent. “Are you crazy? I’ll never climb out of
here.” He tugged himself away from Cassian’s grip—Cassian released him
after a moment—and adjusted his sling. “My arm…” He rotated his body
awkwardly to watch the alley mouth.
Cassian heard footsteps and a distant, distorted yell. He looked Tivik
up and down and realized that, in all likelihood, the man was right:
He really couldn’t make it up the wall, not without help and not
swiftly. In the best-case scenario, by the time both he and Cassian
were up on the roofs, the stormtroopers would already have identified
them and initiated a cordon.
“We’ll be all right,” Cassian said. And for the third time that day, he squeezed the trigger on his blaster. He heard the sickly electric squawk, smelled burning fibers and worse as Tivik fell to the ground. The informant let out one last little groan, like he’d been troubled in his sleep, and lay still.
They would’ve caught you, Tivik. You would’ve broken. You would’ve died. And neither of us would deliver your message.
Rogue One: A Star Wars Story
The Junior novelisation was even more explicit about his reasoning
A third stormtrooper appeared at the far end of the alley. Rather than charge in to be shot, he called for backup. “Troopers down. Section nine.”
Cassian scanned the alley for a means of escape and spotted some easy handholds he could use to climb out. With Tivik’s damaged arm, though, he’d never make it.
Cassian couldn’t leave him there to be captured. The man would crack for sure, and if he told the Empire what he knew…
Cassian had no choice. He leveled his blaster and shot Tivik dead. The stormtrooper stood there, stunned, and watched the man fall.
Rogue One: A Star Wars Story – A Junior Novel