Right Speech & Precepts
You can criticize if it is beneficial to you and/or others, but you should not get angry.
You can advice the person not to do so, if you think he will listen. In any case, be mindful of the concern about your ego. You cannot really stop people from talking whatever they like behind your back. Be mindful of the anger arising in you. Do either Metta or Satipattana meditation to deal with anger.
Intention takes precedence always. It's never the act. Stealing is wrong because it is caused by greed and delusion. Regarding sliced bread, taking something is ok if the owner does not mind you taking it. It does not break the precept. You can prevent someone from hurting another by blocking his path or holding him tight. But hurting intentionally usually involves aversion. It's up to you to check if hate/aversion creeps in.