'If you kill people who surrender it makes it easier for you because no one will oppose you' - Shishin the Big Brain Strategist
Nah that's just dumb, the reason people don't kill surrendered enemies is because if they do everyone else learns that all fights are to the death, this is exactly why Sun Tzu said to never corner enemies. His next line "those waiting on the sideline will surely SURRENDER to us one after another" OK at least this I think actually means "will surely JOIN/SUPPORT us" so he's not contradicting himself his stance is to win over/threaten the neutrals.
Still it will mean that everyone on Gaira's side will fight more fiercely that's the trade off.