Thank you. That's a weird piece of drafting. The first part permits the use of force (even if wholly disproportionate so long as not deadly) but ONLY in response to a threat of unlawful force or reasonable fear of same.

So arguably Martin was entitled to use non-deadly force against Zimmerman.

But the second part allows the use of deadly force (to prevent imminent death or great bodily harm etc) whether or not that imminent death arises from unlawful force.

That would appear to justify killing or eating the cabin boy to avoid your own starvation. Or shooting the other passengers if there was only one parachute and the plane was going down.

Unless the second part is restricted by the first, in which case assuming Martin was entitled as I suggest above, his force was not unlawful, so Zimmerman could not use deadly force.