Is it permissible to curse in private without anyone knowing, and will a person be held accountable for what they say in private?

It is permissible to criticize a person in a low voice if it does not involve cursing or obscenity. As for cursing in the heart, it is forgiven as long as no injustice or harm emanates from the person being cursed, and you do not harbor ill-will or believe them to be characterized by what you curse them for, or intend them ill. Otherwise, ill-will towards them is forbidden. As for unjust cursing, it is permissible secretly and openly.