The Messenger of Allah ﷺ said, "When you hear a man say, 'The people are ruined,' he himself is the most ruined of them."
حدثنا إسحاق قال أخبرنا مالك عن سهيل بن أبي صالح عن أبيه
أن رسول الله ﷺ قال إذا سمعت الرجل يقول هلك الناس فهو أهلكهم
Its chain of transmission is sahih. Accordingly, it is not permissible for a person to issue a ruling denying the existence of good in people. It is found in Muslim 4/2024, no. 2623, in Malik 2/984, and in Bukhari's Adab al-Mufrad 258, no. 760.