We have not been told the exact circumstances.
We do know he was in his bedroom, so he could not have been "running away."

We can be sure that the possibility of an explosive belt was considered.
In any case, a combatant can legally be killed unless actually surrendering.
Either one of those factors would justify killing him.

We have been through all this before, so what is your actual objection?

"flushed with victory "
That is the salient point.
That war was over. They were no longer combatants.