You're extrapolating quite a bit from a simple comment, which tells me that you'd probably be a poor manager as well. Then again, I'm extrapolating quite a bit as well.

Seeing something like JSON.parse throughout the code is definitely a code smell and could decrease the maintainability of the codebase, and that's a very tangible problem. Obviously you shouldn't fire someone over something like this if it's the first offense, but it definitely raises red flags and should make you monitor things a little more closely. If they show a pattern of dogmatism and poor judgement, you're probably better off finding someone else with better judgement. You're not going to find a perfect employee, but some employees are just better at making decisions for a larger project than others.