Final answer:
The principle violated was Least privilege, as the administrator had more access than necessary for his job, which enabled unauthorized changes and installation of a malicious script.
Step-by-step explanation:
The basic principle violated during the administrator's employment is Least privilege. The principle of least privilege mandates that users, including administrators, should only have the access that is necessary for them to perform their job functions. In this case, the administrator retained privileges from all previous roles, which gave him more access than necessary for his current job function. This excess in privileges allowed him to make unauthorized changes and eventually install a malicious script as a retaliation tactic after termination. To mitigate such risks, organizations should routinely review and revoke unnecessary privileges, following a strict access control policy.