Last month, the BBC revealed 15 staff members had been arrested since October 2024 following allegations of rape, ill-treatment and neglect.
Patients, relatives and whistleblowers described a culture of abuse at the unit.
Ten people remain under suspicion and have been bailed or released pending inquiries.
St Andrew’s chief executive, Dr Vivienne McVey, announced her departure in an email to staff on Monday afternoon – in a move that Samari described as “deeply disappointing”.
“I think it sends a really bad signal to our members, to patients and service users, and their families,” he said.
“We are shocked by the failure to take any responsibility or accountability by the chief executive who, instead of taking that accountability and responsibility for management failures, emphasised instead how she was immensely proud of her time at St Andrews in her parting note.”