A person is justified in using force, except deadly force, against another when and to the extent that the person reasonably believes that such conduct is necessary to defend himself or herself or another against the other’s imminent use of unlawful force. However, a person is justified in the use of deadly force and does not have a duty to retreat if:(1) He or she reasonably believes that such force is necessary to prevent imminent death or great bodily harm to himself or herself or another or to prevent the imminent commission of a forcible felony; or
(2) Under those circumstances permitted pursuant to s. 776.013. If Zimmerman was facing "great bodily harm" (like brain damage) from the beating, then he was justified. Period.