As a junior, I worked with a senior who was considered "10x" and working with him was a pain: he decided the rules didn't apply to him, and management tolerated

As a junior, I worked with a senior who was considered "10x" and working with him was a pain: he decided the rules didn't apply to him, and management tolerated his repeated violations of conventions instituted to make our dynamic-language codebase manageable. Anytime he "improved" a module, no one else could maintain it as that would entail additional rule-breaking, which was verboten for mortals, so only he could maintain code he touched. Combined with the fact that he didn't add any tests: the net result was he was slowly and surely subverting the codebase into his personal, brittle domain that no one else could change. He was slowing everyone else done, but all management was looking at was his velocity at closing bugs or rolling out new features while creating tech-debt. His boastful personality was just the icing on the cake.