Stronger and more formal than
离开 ('leave') or
脱离 ('break away from').
叛离 carries an unmistakable moral judgment — it implies betrayal of trust or allegiance, not just departure. Common collocations:
叛离组织 'defect from the organization,'
叛离国家 'defect from one's country,'
叛离信仰 'abandon one's faith.'