Assalamu alaykum warahmatullahi wabarakatuhu. Is it a sin to feel contempt or disgust towards filthy, smelling, untidy, unnmanerly people? Is not it a part of fitrah (to feel that)?

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.

Merely feeling disgusted by those with a bad smell is not a sin unless it leads to harming others or belittling them or despising them. Islamic texts indicated that people are harmed by bad smells, and in order to avoid their harm one may refrain from meeting with them in the mosque.

An-Nawawi said in al-Majmoo’: “It is hated for those who eat garlic, or onions, or leeks, or other things that have a bad smell, and its smell remained, to enter the mosque without necessity, because of the authentic Hadiths about that, including the Hadith of Ibn Umar that Prophet said: “He who has eaten from this tree - meaning garlic – should stay away from our mosque.” [Al-Bukhari and Muslim] The wording by Muslim reads: “… our mosques.” Anas narrated that the Prophet said: “Whoever eats garlic or onion should keep away from us or our mosques.” [Al-Bukhari and Muslim] Another narration by Muslim reads: “He who has eaten onion or garlic, or leek should not approach our mosque, because the angels are also offended by that which offends the children of Adam.”

The same thing applies to feeling disgusted with the rude and harsh people. Allah stated the favors that he granted to His Prophet of being merciful and avoiding rudeness and warned him that if he were rude people would have disbanded from about him, as Allah Says (what means): {So by mercy from Allah, [O Muhammad], you were lenient with them. And if you had been rude [in speech] and harsh in heart, they would have disbanded from about you.} [Quran 3:159]

Ibn Kathir said in his Tafseer (the interpretation of the Quran):
“It means that if you were rude in speech, harsh-hearted against them, they would have broken away from about you, but Allah gathered them around you; so be lenient with them as a way of softening their hearts.” [End of quote]

Allah knows best.