What I think could be the case is that when doing entries the one and two man are the primary shooters followed by the 3 man skirting the primary and support roles and then the 4 man mainly as support in the room if he can get in. 1 and 2 man are surely going to engage and the 3 man will with whoevers sector he is in because he is going to decide follow one of the two front members and the 4 man will follow the other.
That's by the book for actual room clearing and this was not by the book, but the basic fact remains that the one and two man will engage first and three man will typically engage as well so that three man was ready to engage regardless. I think when he came in slightly late he only saw the guy crouching behind the counter and he didn't see exactly what the one and two man did so he perceived it completely different and still perceived the guy as a threat. If you judge it by the book then he was doing what he was supposed to do and it just looked like he took a late shot.
