I am a native British English speaker and I would say definitely without the 'in'.
It would also be slightly more elegant to avoid splitting 'cutting' and 'off'. So:
Cutting off the commander-in-chief mid sentence..
Also - just to be super picky and technical, it doesn't make a lot of sense to include the word 'reportedly' later on. Reportedly literally means 'according to someone else's report', i.e. the narrator wasn't there. However the first half of the sentence is very specific about the details of the General's interruption, and sounds like the narrator is there. It would be better to say either:
Cutting off the commander-in-chief mid-sentence and demanding that he give them a straight answer, U.S. General Paul Selva snapped “Look, just tell us who to kill”
This means the action unfolds in the present and you feel like you are part of the story
The U.S. General Paul Selva reportedly cut off the commander-in-chief mid-sentence, demanding that he give them a straight answer, and snapped “Look, just tell us who to kill”
That sounds more like someone is explaining (reporting) some actions that happened in the past.