What is the ruling on rejoicing inwardly at the killing of a Muslim by a disbeliever, or at harm befalling a Muslim, and does such rejoicing constitute apostasy from the religion?

You must ignore whispers (waswasah) concerning apostasy and pay no attention to them, because they bring evil to a person. Furthermore, rejoicing at a Muslim's harm is blameworthy in Islamic law, but it does not constitute disbelief as long as one strives against this feeling.