You don’t need religion to be a moral person, and you don’t have to reject religion to act amorally. But there is no perfect, universal, scientific morality. Cultures, communities, individuals will vary on what they consider a moral act, and morality can change with circumstance. When different moralities interact, there will be conflict. And the amoral (or rather those, who do not subscribe to the same morality as those around them) will always use others’ morality as a tool to manipulate, a curtain to hide behind, a weapon to wield, and a shield to defend with.
Religion helps communities to build a common morality in order to reduce tensions and foster fellowship within the group. But there will always be communities. There will always be disagreements, confusion, frustration, pride, loyalty, forgetfulness, honor, greed, hunger, struggle, disease, countervailing needs and desires, and mercy. The absence of religion would not stop people from seeking safe harbor and kinship in others, whether that is social clubs, fandoms, sports teams, political parties, activist organizations, etc. And when that kinship is endangered or perceived to be endangered, the absence of religion will not stop people from seeking to obstruct, forestall, eliminate, or revenge against whatever or whomever is perceived to be the cause.