All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
It is not permissible for a Muslim to despise any other Muslim or to look down on him, because the superiority in the Sight of Allah is the piety that is in the heart. Only Allah knows what is in the hearts. Looking down on one's fellow Muslims is due to arrogance, which is one of the grave major sins.
It is confirmed that the Prophet said, “The one who has the weight of a mustard seed of arrogance in his heart shall not enter Paradise.” They (the Companions) said, “Verily, a person loves that his dress should be fine and his shoes should be fine.” He (the Prophet) remarked, “That is not arrogance. Arrogance is rejecting the truth (out of self-conceit) and having contempt for people.” [Al-Bukhaari and Muslim]
If you find someone who has this bad quality, then you should advise him and explain to him that he is in great danger and that it is not permissible for him to despise the Muslims or look down on them, as perhaps the person whom he looks down upon is better than him in the Sight of Allah. Hating the sinfulness of that person and hating his arrogance is considered part of the religion. A person may be loved for an aspect of his character and hated for another aspect, as has been clarified by the scholars. So a person who has this quality of arrogance is hated because of this dispraised quality that he has without any doubt.
For more benefit, please refer to fatwa 19854.
Allah knows best.