This guy is worst than Rimuru.
After seeing the cruelty and the necessity of killing one's opponents Rimuru threw aside his previous feelings to protect what was important to him.
This guy saw first-hand how irredeemable some of these humans are and STILL insist on not killing humans knowing that they will just come back again.
That's just high lvls of softness and stupidity. If he met the guy who murdered him again, I doubt he even does anything to him.
In a attack, not killing your enemies can have advantages. You can still gather good will, eliminating some of the hatred if you occupy that area.
During defense though, not killing your enemies just means...well they gonna come back and little by little exhaust your troops.