"What about the brutal sack of Carthage,"
The surrendering population wasn't murdered.
"or all of the Gauls Caesar killed for no reason other than to make himself look good?"
What are you talking about? This is how Caesar treated the Gauls.
"The Helvetii, the Tulingi, and the Latobrigi to return to their territories from which they had come, and as there was at home nothing whereby they might support their hunger, all the productions of the earth having been destroyed, he commanded the Allobroges to let them have a plentiful supply of corn; and ordered them to rebuild the towns and villages which they had burnt. This he did, chiefly on this account, because he was unwilling that the country, from which the Helvetii had departed, should be untenanted, lest the Germans, who dwell on the other side of the Rhine, should, on account of the excellence of the lands, cross over from their own territories into those of the Helvetii, and become borderers upon the province of Gaul and the Allobroges. He granted the petition of the Aedui, that they might settle the Boii, in their own (i.e. in the Aeduan) territories, as these were known to be of distinguished valour to whom they gave lands, and whom they afterwards admitted to the same state of rights and freedom as themselves."
http://classics.mit.edu/Caesar/gallic.1.1.html
Even after the final conquest of Alesia Caesar did not murder the population, but sold them into slavery, even pardoning some. Nothing comparable to the Mongols.
Just realized
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