When an employee consistently targets a teammate and continues to criticize his, but not criticize other teammate's shoddy work, what idea of his should I be cr

When an employee consistently targets a teammate and continues to criticize his, but not criticize other teammate's shoddy work, what idea of his should I be criticizing? When an employee consistently commits to doing work and then fails to later on, justifying it (post-hoc, after the deadline) with "I think we shouldn't go in that direction", what idea of his should I criticize? I could say that I can criticize his failure to meet his commitments, but we're just mincing words. In common parlance, that's equivalent to criticizing him.