Sometimes, sadly, we hurt people we care about by our actions or inactions. When we become aware we’ve caused pain to someone we love, the only thing I’ve figured out to do is acknowledge causing the pain, take responsibility for acting badly and sincerely ask for forgiveness. I don’t know of any other way, though some people buy gifts, take special care of the person they hurt as a way of making it up to them. Not everybody is capable of taking responsibility for cruel things they do in anger.
Anger itself is partly to blame, it is a famously terrible emotion and as difficult to sit with as grief. When you’re angry you can only see the thing that makes you angry, in vivid black and white — no gradations of any kind. When you’re mad you can’t see the harm your anger may be making you inflict. You let the arrow loose from the bow in an act of righteous anger and it finds its mark, and even if it doesn’t inflict a fatal wound, it can still kill.
If somebody shows you their shock and anger when you ask to discuss something they did that hurt you, believe what they are showing you. They are not capable of anything beyond that. Believe them.