How do I deal with a talented employee who hurts the team?

You have someone who delivers brilliant results and quietly poisons everyone around them, and you avoid confronting it because of what they produce. You know you are trading the whole team's health for one person's output. Naming it that plainly is the first honest step, because the trade only continues while you pretend it is not happening. Do the real math. One person's brilliant output does not offset the drag on everyone they demoralize — the good people who go quiet, disengage, or leave. You are not keeping a star; you are letting a slow leak sink the boat because the leak is also fast at rowing. And every day you tolerate it, you are teaching the whole team what you actually reward: results, at any cost to each other. So have the conversation. Be clear and specific — the results are valued, the behavior is not, and the behavior has to change. Give them a real chance to hear it and adjust. But make plain, to them and to yourself, that no output buys a pass on how people are treated. Your team is watching to see whether that is true.