Just a hypothetical question: would those who are calling this act "murder" oppose the reprisal had it been authorized by the theater commander (assuming it had been done as a "last resort")?
Sounds a lot like the Malmedy Massacre on a smaller scale, which militarily was a "last resort" because the prisoners would have held up the SS advance on Stavelot (just down the Ambleve from Malmedy). Thus, a War Crime on the part of both the shooter and the theatre commander (IMO).

Changing the facts (adding more facts) might give a more nuanced response.