The whole point of "professional" discourse is to keep emotions and feelings out of it so the substance can be the focus. If you start insulting people then it is going to trigger an emotional response. That's how most humans work. If you choose not to be careful about how you word your criticism, then you are also choosing to inject emotion into the conversation, thus distracting from the substance.

You can be blunt and direct without being an asshole. It's not that hard.