1. Fatal Sins in Islam
Calling Other Muslims Kafr, Intentionally Harming another Muslim, Insulting those who are close to Allah, Arrogance

2. Fatal Sin Forty Nine: Rebelion and Calling Muslims Kafr
Allaah says :
But do not transgress limits, for Allaah loves not transgressors.
If any one disobeys Allaah and His Messneger, he is indeed on clearly wrong path.
The Prophet said : He who says to his brother Kafir : one of them will bear it

3. Fatal Sin Fifty: Hurting Muslims and Insulting them.
Allaah says :
And those who annoy believing men and women undeservingly bear (on themselves) a calmuny and a glaring sin.
And spy not on each other, nor speak ill of each other behined their backs.
He also says :
O you who believe !! Let not some men wrong you laugh at ohters It may be that the latter are better than the former.
A third verse is :
Woe to every scandal monger and backbiter

4. The Prophet said : The worst rank of people to Allaah are those whom people act goodly towards him to avoid his anger. The Prophet also said : Allaah hates those who are obscene
The Prophet said:
Slaves of Allaah ! Allaah has created embarresment but not defaming your brothers, he who does so will be destroyed. The Prophet also said : the Muslim is forbidden on the Muslim : his honour, money and blood. Virtue [piety ] is here [Pointing to his chest].
The Prophet said ;
A Muslim is the brother of a Muslim, he must not unfair him nor let him down nor dispise him. It is enough of evil to dispise his Muslim brother.
Allaah says :
Those who love to see scandal published broadcast among the believers, will have a grievous penalty in this life and in the Hereafter.

5. The Prophet also said : He whom his nieghbour does not trust does not enter Paradise.The Prophet also said : By Allaah he does not believe - He whom his neighbour does not trust his evilness
It was said to the Prophet, O Prophet of Allaah! There is a woman who prays at night, fasts during the day, but by her tongue hurts others. The Prophet said : There is no good in her, she is to hell.
The Prophet said : when I was ascended to the Heavens I saw people with copper nails scratching their faces and breasts. I asked : O Jibrail Who are they ? he said : They are those who eating peoples flesh and defaming them.

6. The Prophet said: It is one of the fatal sins insulting the parents
The Prophet said: It is one of the fatal sins insulting the parents. They said Does a man insult his parents? The Prophet said : Yes. He insults others parents and the latter insults his.
In another narration :
It is one of the fatal sins cursing the parents they said O Prophet of Allaah , how a man curses his parents? The Prophet said : by insulting others father and mother, then the latter returns back the insult for him.
The Prophet said : no man calls the other blasphemer or sinner for if the latter is not so, these attributes will turn back to him.
The Prophet also said : Do not insult the dead for they have told me what they have done .

7. Fatal Sin no 51: Insulting friends of Allaah and being Hostile to them
Allaah says :
Those who annoy Allaah and His Rasool Allaah has cursed them in this world and in the hereafter.
The Prophet said that Allaah says :
He who insults a friend of mine I will declare war in him. In another narration he competes in war In the hadeeth O Aba Bakr ! If you made them angry you made Allaah angry. Them refers to the Muhajireen

8. Fatal Sin no. 52: Lowering clothes out of Pride (for Men) – Walking the earth looking down on others (for women and men)
Allaah says :
....Nor walk in insolence through the earth
The Prophet said : Whatever cloth is beyond the ankle is in the Hell. He also said : Allaah does not look at the one who drags his clothes out of pride. The Prophet also said : Three are those whom Allaah will not look at nor praise and will receive painful punishement : the lowerer of his clothes, the reminder of charities and one who sells his goods with false swearings.
The Prophet also said : Once a man was walking wearing a suit which he was proud of his hair is combed and walking proudly, Allaah destroyed him by allowing the earth to swallow him up, and he will remain rattling there until the day of Judgement.