Just to clarify that a person can be Jewish and be against Zionism. These two terms are not synonyms.
Jewish Voice for Peace have a great article on the topic and it includes the following statements:
Jewish Voice for Peace is guided by a vision of justice, equality and freedom for all people. We unequivocally oppose Zionism because it is counter to those ideals.
While it had many strains historically, the Zionism that took hold and stands today is a settler-colonial movement, establishing an apartheid state where Jews have more rights than others.
Palestinian dispossession and occupation are by design. Zionism has meant profound trauma for generations, systematically separating Palestinians from their homes, land, and each other. Zionism, in practice, has resulted in massacres of Palestinian people, ancient villages and olive groves destroyed, families who live just a mile away from each other separated by checkpoints and walls, and children holding onto the keys of the homes from which their grandparents were forcibly exiled.