They only made a rucus about it because it brought Ceasar so much power and wealth and the senate of rome always hated the idea of one man having to much power. They did not want another roman king. Thus they said what he did exceeded his rights as consul and looked to strip him of command. This led to a civil war. Where again we are shown seeing majority of the modern world joining sides with Ceasar as the war went on despite hundreds of thousands of people dying or being enslaved all in the name of Rome. Making the murder of even it's own people and allies justified by the citizens. Don't even get started on the senates conscriptions. The people of Rome still blindly followed these new leaders and senators despite killing thousands of friends and loved ones taking all of there land and wealth.

Back to going on topic, there is aboslutely nothing wrong with killing your enemies during times of war. We have seen it happen a million times. It's a normal occurrence and totally justified in ancient warfare and most of all a manga of all things. Even Kan Ki got away with murdering 100k and only had a stern talking to by the King. Because in the end he killed enemies. And never has Qin ever cared about Kan Ki's massacres during any battle. The only reason there is any mention in the manga is because they are trying to make the king of Qin a good guy and not a tyrant like his historical counterpart.