It's just my interpretation of the scene, but it appears to me that the killer may have had more time than Asmo to act.

Asmo opens the door outwards into the hallway, then he takes one step and stops, says "You? No!" and is killed before the sound of his words fade.

We may be overlooking that the killer may have seen Asmo as early as his opening the door. If it is a channeller, perhaps they then had enough time to channel a weave that hit him while he was speaking?

Perhaps Asmo didnt see what was happening until whatever was weaved came at him?

The timing works.

i agree it wasnt BF.