I think that's a deliberate decision on Paramount's part. The Bumblebee movie moved away from the Bayformer formula, but commercially underperformed. The Bayformers movies were commercial successes. At the end of the day, we all vote with our wallets. For my part I watched Bumblebee thrice in cinemas.
Optimus Prime is supposed to start from a dark place, having been stuck on Earth for seven years and desperate to get back to Cybertron to continue the war against the Decepticons. I like how the movie started Optimus Prime from a low point so that he can grow as a character. I also liked how both Noah and Optimus Prime had parallels as the respective "big brother" characters for their respective families, which is something that helps both characters grow. Noah chooses to spare the Key to save Prime's family, while Prime destroys the Key to save Noah's.
Yes, Optimus Prime did brutally kill Scourge, but this was in the middle of combat. I personally don't object to him killing enemy combatants during a fight; that's just the awful nature of war. What I object to is when he kills opponents after the fight has ended; i.e. executing POWs - a war crime. This isn't what Prime did to Scourge though. It's not comparable with the way that he executed Demolishor in RotF or Sentinel Prime in DotM, IMHO.