The Talmud teaches, “All Israel are responsible [areivim] for each other”. Rashi explains that we have a duty to respond when we see others doing harmful things. Jews have a common spiritual identity and destiny.
What about minding your own business and not judging others? Rashi explains that God does not hold us responsible for the hidden deeds of others. I read this to say if there’s a way to give someone the benefit of the doubt — perhaps there is a hidden perspective that makes a certain act acceptable — then I shouldn’t interfere. It’s not our job to go digging for others’ wrong-doing but if it’s blatant, there is a case for speaking up and trying to stop it.
The duty of mutual responsibility extends also to ensuring that others’ basic physical needs for food, shelter and security are met and also to helping meet their spiritual needs, for example by fulfilling certain mitzvot on their behalf, such as making kiddush.