lol In almost all cases, except for one notable exception (Temujin/Genghis Khan).
Portrayed as a blood-thirsty, vicious, brutal man. Because his history was left to be told by the defeated.
Actually a very honorable man, strong family values, exceptional military mind, exceptional social mind.
Although, it's said that this was a tactical decision, in line with Sun-tzu's writings.
"The greatest victory, is to win without needing to fight."
If you wipe out the defending army, no-one lives to tell your tale.
If you leave survivors then they can go to the next town and tell tales of your victory.
+Chinese whispers.
When you reach the next town, they may not even want to fight, if the stories grow horrible enough & they become terrified of what you'll do.
Fear is a potent weapon.