Is the work of a private security guard forbidden (haram)?

The work of a private security guard falls under the contract of ijarah (leasing/hiring) and takes its rulings. If he guards something permissible, such as a market, fruits, or schools, then his work and wages are permissible. However, guarding usurious banks or places of forbidden entertainment makes his work and wages unlawful. Guarding individuals is permissible in principle, and it may be obligatory, recommended, or forbidden. So, whoever hires himself out to an oppressor or usurper to assist him in his oppression and usurpation, that would be unlawful.