to be fair the guy was probably just doing his job. if you had the choice between killing a few primitive creatures i.e. ants and rebelling against a planet buster only to become meat giblets, which would you chose.

come to think about it, I kind of hate sweet mask even more now.

I remember how all the tsukommus in Mangahere were arguing over whether Sweet Mask killing the surrendering aliens was right or wrong. Looks like the anime did a good job of making the aliens that served Boros more sympathetic and their deaths more sad as a result.