I understand the point 1 and 2 but i dont really understand the 3rd point, I have watched the video several days ago but I still dont understand the details of going astray, does it mean even the most pious one of sunni school of thought could do evils? The possibility of them doing evils is higher than that of the followers of shia school of thought?

how about the way their salah is different, do the rewards of their salah become less? even they do it full of concentration?

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They may be pious, but they are not worshipping Allah the way Allah wants. They are not praying according to what Allah wants. So Allah may not reward them for it.
Also, the follow figures who were enemies of God and Ahlulbayt. That’s a major offense.
Remembering the Kharijites looked pious from the outside but the Prophet condemned them. This doesn’t mean all Sunnis are bad. Those who know the truth and are stubborn will be punished but those who are not stubborn and they really lived a life of goodness and they don’t hate the Ahlulbayt, then the intercession of Prophet Mohammad (s) can include them.