What good is showing regret? They're at war with the Vandenreich and have already lost well over a hundred, one of which was a Vice Captain. There's no time for regret or morality. They have to keep the balance intact and find some way to stop the Vandenreich continuing to cause the imbalance. They can mourn the gone (can't really call them dead) later.

It wasn't a case of right or wrong - prolonged imbalance between the number of the Souls and the Hollows will destroy everything. Humans, Souls and Hollows will all disappear. Mayuri did the right thing.