Constructive criticism should focus on the issue not the person themselves ie. You could do a better job washing the dishes.

Insults are attacks ie. You can't do anything right, You're lazy and you'll never amount to anything.

Notice the former is honesty, while the latter is abusive.
Telling someone that they could do a better job is giving them the opportunity to fix their mistake. Name calling and judgements label them, which no one has a right to do.