It probably wasn't black-and-white like that. Just as there were pro- and contra-Macedonian factions in most Greek cities, so there would have been amongst the mercenaries. Similarly, Alexander could be ruthless or merciful (or, at least, willing to recruit them) depending on the circumstances. One chronicler wrote that Alexander would always forgive Theban mercenaries for fighting against him (because of what had happened to Thebes).

The Greeks that remained with Darius after Gaugemala were the die-hards. They only surrendered after Darius was killed. Alexander may have chosen to recruit them because it was faster than waiting for reinforcements from Macedon, and the men were already battle-hardened,