I don't think killing the super human assassins that were made with his blood is what it showing his deteriorating humanity
To me setting up the paramedics to get killed in a fake out, a completely unrelated party that went there to help him, and then using them just to lure the assassin is a better example of how ruthless he is becoming, he knew the whole ambulance was going to be targeted and still went with it
"how long would this bloody conflict last?" thinks koichi, the walking killing machine whose life's purpose is making his enemies's lives absolute hell