When it comes to Fitz, Mack has every right to criticize Fitz's behavior. In terms of the whole 'killing' issue, I'd blame the writing on that. I could understand Mack being hesitant about killing due to being a mechanic who was being roped into special ops, but Daisy was tasked with killing Donnie in season two when he became a threat, so killing people is part of the job. It was even suggested to kill Mike during the premiere episode, so it's not like the organization doesn't kill (I know Coulson avoids it if possible at times, but we're not dealing with a superhero group who has a 'no kill' policy, like in some comic books).
I think part of the issue is a balancing act between this technically being a 'superhero' show (due to being set in the MCU) and the team being part of a spy organization that necessitates some hard calls at times, but I don't think the writers have the best grasp on what to do or how to handle it. Certain characters and plots get dropped (like Agent Weaver, the Council, Vijay), and the writers get confused (like bringing Fitz and Simmons together at the end of season two without addressing any of the issues that divided them that season, or Daisy thanking Fitz at the end of season four for something Mack actually did).