Aqaaid

Q: I want to know the ruling for people leaving Islam. Should we kill them? Also please tell me what Quran says about a murtad? Why do people leave Islam eventhough they know the truth?

Q:

1. If someone does kufr (by mocking islam) after that he comes to know its kufr, is it necessary to say shahadah and repent?

Case1: I am asking this because if a person doesn't know its sin, he won't be getting any sin right? Then why is shahadah and repentance required (if he is not gettig sin).

2. If I said to someone that "making fun of islam is kufr and whoever does this is, is a kafir and needs to repent and say shahdah again to become muslim." Instead of knowing the correct ruling, i just purposely told him to do repentance and say shahadah again to become muslim. Did i commit kufr by making halal haram by telling someone to do an act which is not required and telling them only if u do this then you become muslim. 

Q: If a Muslim has a doubt concerning an Islamic thing, like he heard someone say something about Islam but its widely accepted thing and he gets doubt that if its true or not. If true where is the proof? Like these thoughts. Now I he gets free time and wastes it. Will he be regarded as a kaafir?

Q: This question is very important. In fact, almost all committing this sin face this, but do not think about it and do not ask about it. Will this be a kufr to watch pornography and masturbate, in which actors pronounce the name of God during adultery and/or they utter phrases that offend God, during adultery? Will this be contentment with unbelief and with an insult to God, if muslim does not agree with these words and actions in the heart?