(As a companion to DVK's answer based on the novel, here's an answer sourced only from what's shown in the film.)

Cassian's Orders Are Re-Confirmed Cassian does check in with his superiors after the destruction at Jedha. He tells them about Jedha and about Galen Erso being at Eadu.

General Draven replies with a reiteration of his previous orders — to kill Galen instead of extracting him as a defector.

Even though Draven knows the Death Star is finished, he is concerned about future work that Galen may do for the Empire. He sees him as too dangerous to let live.

Draven says this out loud in the presence of the communication officers who are in contact with Cassian. It's not shown whether the operators pass on the reasoning behind the orders as well as the orders themselves, but either way it's very likely that Cassian understands Draven's reasons quite clearly.

This makes Cassian's later decision to disobey the orders even more significant.

(As a companion to DVK's answer based on the novel, here's an answer sourced only from what's shown in the film.)

**Cassian's Orders Are Re-Confirmed**

General Draven replies with a reiteration of his previous orders — to kill Galen instead of extracting him as a defector.

Even though Draven knows the Death Star is finished, he is concerned about future work that Galen may do for the Empire. He sees him as too dangerous to let live.

Draven says this out loud in the presence of the communication officers who are in contact with Cassian. It's not shown whether the operators pass on the reasoning behind the orders as well as the orders themselves, but either way it's very likely that Cassian understands Draven's reasons quite clearly.

This makes Cassian's later decision to disobey the orders even more significant.