When we recite Durood (Salawaat) we mention the Prophet Muhammad (SAWA) and his Progeny (Ahlul Bayt)(AS) and Hazrat Ali (AS) is the head of Ahlul Bayt (AS) after the Prophet (SAWA).

Imam Ali (AS) was brought in Mubahala as Nafs of the Prophet (SAWA) while Hasan (AS) and Husain (AS) where brought as the sons of the Prophet (SAWA).

Imam Redha (AS) when Al-Ma'moon asked him about he greatest tribute of Ali in Quran, said : Mubahala when Allah mentioned Ali as Nafs of the Prophet (SAWA).

Wassalam

Allah out of His love and mercy tells us what will happen if we do certain things in order to warn us - it's like as an adult I know that if I throw a rock up it will fall back down due to the law of gravity - a child may not  know this - as a loving parent would I not tell my child that if you throw the rock up it will come back down and hurt you.

There is a system of cause and effect and Allah is explaining this to us. So we know if we do somethin what the effect of it will be. 

It is important to fear disappointing Allah rather than fearing Him. 

According to Islam all humans are the servants of Allah and slavery is wrong. However the time of Prophet a lot of people were slave and slavery was acceptable in the world. Islam started to teach people that it is wrong and to free the slaves step by step . First Islam gave them rights and made rules to support them.... 

Anyway according to Islam the female slaves were like the wife of the owner and the male slaves were mahram. 

If he started propagating against Islam, then he/she will be punished.

Verse 3:86 & 87 in the Quran are about irtedad but it is not about punishment. It is in ahadith.