It is problematic topic and it depends on field of view. Anyway,you need to take into consideration that most of the time, killing all these people was not the prime intention of those historical figures. for example Napoleon was not fighting to wipe out his enemies, even if its quite popular to compare him to Hitler.. he was quite far from it. If there was no Napoleon, war would happen anyway.. European Absolutist Monarchs considered war as the only possible way how to solve things, and this lead to more deaths than you can write to Napoleon (World War I, anybody?) On contrary, Napoleon initial intention was to spread French system to other countries that were still locked in absolutism, yet he didn't understood that some people are way too simple and quite too easy to manipulate to understand it (Spain, Russia)
But back to Romans and Caesar. It is not like Gauls were some kind of a peace loving community that never ever attacked anybody... no. they launched lots of attacks against Rome, and were considered a constant threat (to everybody - look at what happened to Macedonia when they underestimated Gallic threat - Ptolemy Caraunus,Macedonian King at the time of Gallatian invasion, was captured, beheaded and his head was impaled on a lance..). You never knew when there emerges another Gallic king that gets all men he can get and march south... Caesars actions were aimed at eliminating that threat once and for good. Did he went too far? probably. yet, i would say it was out of necessity, considering his previous campaigns in Gaul - He subjugated Celtic tribes one by one, took hostages to make sure they stay loyal (common practice of those days) yet, he had to face several betrayals of those tribes, some quite dangerous and close calls (they sieged all his winter camps across the Galia at once, so he could not come to help)..
If you look at that context, his later actions during Vercingetorix uprising were necessary from his point of view. it was practically the only way how to prevent Gauls to rebel again, and he succeeded in that. there were no major revolts in Galia after this...