To arrest someone for a crime, the police need probable cause to believe that he committed the crime. But what if it’s clear that the person committed the act (e.g., intentionally killed someone), but it seems likely that he has a good affirmative defense (e.g., self-defense)? My view is that probable cause should be probable cause to believe that the conduct was indeed criminal, and if the self-defense case is strong enough, that negates probable cause to believe that a crime (as opposed to a justifiable homicide) was committed. But when I looked into this several years ago, I saw that the few courts that had discussed the matter were split.
Florida law, though, clearly resolves this: “A law enforcement agency ... may not arrest [a] person for using force [in a self-defense situation] unless it determines that there is probable cause that the force that was used was unlawful.”
So in Florida, the police must have probable cause to believe that the defendant wasn’t acting in lawful self-defense in order to arrest the defendant. It’s not enough to say, “we have probable cause to believe that you killed the victim, so we’ll arrest you and then sort out later how strong your self-defense case is.”
I can’t speak with confidence to whether in the Martin/Zimmerman case the police indeed have such probable cause (which, as you may recall, is a not very clearly defined standard that is well below proof beyond a reasonable doubt, and probably somewhat below preponderance of the evidence).