I may be an underperformer in your team but your thoughts (and words) towards me as my leader or manager will be the ceiling for my potential. You call me a muppet (or worse), even in my absence, I’m unlikely to change. By all means talk to me about my behaviour and attitude, but maintain respect for me as a person. And consider this- if my attitude has taken me years to develop, it’s unlikely to permanently change overnight- what input could you give me as a supportive manager (that believes in my potential) to undo years of unconstructive thinking and replace it with a new operating system?