Assalamualikum, Allah (swt) drowned the people of Noah, was the sole reason disbelief in Allah? I would also like to know the nature of these people, is there something that they used to do wicked, evil?

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.

Allah, The Almighty, destroyed the people of Nooh (Noah) due to their disbelief in Allah, first and foremost, in addition to their other sins. They went to extremes in transgression, disbelief, and injustice, as Allah, The Almighty, described them at the end of Soorat An-Najm, Saying (what means): {And the people of Nooh before. Indeed, it was they who were [even] more unjust and oppressing.} [Quran 53:52] It means that they were [even] more unjust and committed graver acts of transgression than the peoples of Thamood, ‘Aad, and others. They disbelieved in the Pprophethood of Nooh ridiculed him, and described him as a man in manifest error. They used to exhort one another to disbelieve in what he communicated to them to such an extent that a man would take his son by the hand, go to Nooh and say (to the son): “Beware of this man; he is a liar. My father took me to him and told me the same thing I am telling you.” Thus, the old people died upon disbelief, and the young children grew up abiding by their fathers’ advice. They exchanged advice amongst them to hold on firmly to their false gods and they arrogantly refused the truth. They persistently held onto their disbelief and devised many evil plots against Nooh and the believers. Whenever Nooh invited them to believe in Allah, The Exalted, they {put their fingers in their ears} to prevent themselves from hearing his exhortation and speech, and {covered themselves with their garments} to prevent themselves from seeing him, may Allaah exalt his mention. Allah, The Exalted, informed about their actions in Soorat Nooh when He Said (what means): {Because of their sins they were drowned and put into the Fire, and they found not for themselves besides Allah [any] helpers.} [Quran 71:25]

The verse uses the plural ‘sins’. Ibn ‘Attiyah said in At-Tahreer wa At-Tanweer: “The plural ‘sins’ refers to Shirk (ascribing partners to Allah), disbelief in the Messenger of Allah, harming him and the believers, ridiculing him when he warned them of the deluge, and all the crimes and immoral acts entailed by those actions.” [End of quote]

Ibn Katheer said in his Tafseer: “Allah, The Exalted, Says (what means): {Because of their sins they were drowned...} [Quran 71:25] It means that they were drowned because of their numerous sins, insolence, persistence in disbelief and opposition to their Messenger.” [End of quote]

Allah Knows best.