The ancient world was totally different to today and there was a very real brutality about it. It was truly a time to kill or be killed. That doesn't excuse Alexander and Caesar for the war crimes they committed, but it helps us understand why they did what they did, and that almost anybody else in their position among their peers would have done the same. You can't show your enemies any more mercy and morality than they'd show you.
That being said people were still cognisant about who constituted a real enemy, and people condemned their leaders for being warhawks fighting unjustifiable illegal wars. Consider that while the Roman people supported Caesar's campaign in Gaul they had bitterly opposed Crassus' campaign against the Parthians, which happened at the same exact time Caesar was tearing up Gaul. In the end Crassus' soldiers mutinied and gave him up to the Parthians, who chopped off his head then poured molten gold down his throat to punish his legendary greed.
Alexander too had to deal with greater and greater unrest in the ranks the further east he pushed, because they didn't see how Indians were enemies of the Greek world. This culminated in another wholesale mutiny on the banks of a river in Punjab, because the soldiers just weren't having it. They knew the only reason he wanted to invade India was to gratify his own ego, and they weren't going to die over something like that.