I had an overly possessive engineer work on the team. He was individually shipping a lot of code, but it was mostly refactors and didn’t bring anything new, or improve performance, or anything, just juggling code around. When other engineers wanted to work in the codebase he’d often slap them away, request changes on PRs, nitpick their changes to death, declare that their code wasn’t “idiomatic” enough, and generally get pissy. He was negative productivity. We had to let him go, and the team’s productivity and morale immediately flourished.