At first the doc seems harmless, even if he is full of zingers like "I forgot my input is needed only in matters of life and death" (14.15), and "I should never have doubted your knowledge of venom" (4.49). Oh, snap.
Before long, his arrogance escalates to another level. We get that he's a doc and all, but he really should pay more attention to medicine and less attention to coming up with his next one-liner. He and Serena spar a few times, each trying to outsmart the other. (We'll let you guess who the winner was.) When he casually dismisses Serena's pains during pregnancy, though, he costs her the life of her baby.
We don't condone Galloway murdering Cheney as a result, but we can also see why Pemberton and Galloway are so ticked off at the doc. He's supposed to heal people, but instead he lets his arrogance get in the way of doing his job. When this happens to Serena, well, it's a fatal mistake.