For me, Hartley's unlucky. His arm was originally swinging well below the line of SOB's shoulders. Looking at the commentary online, he's being judged by history here.
Which is only right and proper.
He had shown himself to be a consistent thug in his rugby career to date, so when viewing any of his actions that is the only prism to use. The idea that he was swinging his arm to do anything less than cause maximum damage is laughable; that's just the type of rugby player he is.