If a person suffering from obsessive doubts (al-waswas) falls into sin, is it considered that he has acted upon what was in his mind, such as insulting the Messenger (peace be upon him)? And is his action considered an act of disbelief (kufr) that necessitates killing? And does suicide erase sin and make repentance acceptable?

We sincerely advise you to seek refuge in Allah and ask Him to remove your worry and relieve your distress, for Allah answers the one in need. Your eagerness to flee from disbelief and your intense fear of it are proof of your faith, as the Prophet, peace and blessings be upon him, said: "There are three qualities, whoever has them will taste the sweetness of faith... and that he hates to return to disbelief after Allah has saved him from it, just as he would hate to be thrown into the Fire." What happened to you also happened to some of the Companions, and the Prophet informed them that their considering the matter immense was a sign of faith. Therefore, completely turn away from these whisperings and do not pay any attention to them, for Satan wants to grieve you with them and corrupt your religion and your life.