You might be very surprised at how differently ancient people viewed the world compared to yourself.
Sure, no one wants to be killed or enslaved but the societies of these very people behaved in precisely the same way as the Romans and most ancient societies did. These were not peace loving hippies being invaded by alien monsters.
Anyway, Caesar was not dictator when he fought in Gaul. He was a Roman proconsul/general fighting in his provinces the way all ancient Roman armies did, and was more inclined toward mercy than most, although he also fought a much bigger war than most.
In the civil war Caesar was fighting Pompey, the guy who used his military power to control the Roman government for the previous decade. And who would have continued controlling the government had he won. The narrative that Pompey was defending the republic against a tyrant is more ancient propaganda than reality.
A reasonable and accurate summary is that Julius Caesar led a violent army against violent enemies, fought a civil war with unprecedented clemency, and was a benevolent dictator until his assassination by the envious people he had granted clemency.
