People who are convinced that George Zimmerman killed Trayvon Martin for no good reason frequently say it's absurd to suggest that an unarmed 17-year-old could have posed a deadly threat to a 28-year-old man who outweighed him by 100 pounds. According to a story in Monday's New York Times, however, Zimmerman is only 20 pounds heavier than Martin, who was four inches taller.

The revised numbers by no means show that Zimmerman, a Sanford, Florida, neighborhood watch volunteer, is telling the truth when he says he shot Martin in self-defense on the night of February 26. But given that such basic facts are still a matter of dispute, we should be especially cautious about rushing to judgment on bigger questions like these:

Who started the fight? It seems clear that Martin would still be alive if Zimmerman had not taken it upon himself to follow a teenager he deemed "real suspicious" in light of recent burglaries attributed to young black men. But Zimmerman claims he was heading back to his SUV when Martin approached him and, after a brief exchange, punched him in the face hard enough to knock him down. Although Martin was understandably nervous (and possibly angry) about being followed, that by itself would not justify the assault Zimmerman describes. But if Zimmerman did more than tail Martin—if he shoved him or displayed his gun, say—he might qualify as the initial aggressor.

Did Zimmerman shoot Martin "in cold blood"? So claimed Natalie Jackson, a lawyer for Martin's family, citing cries for help in the background of a 911 call received during the fight. "It is so clear that this was a 17-year-old boy pleading for his life," Jackson said. Not so clear, actually: While Martin's relatives say they recognize his voice, Zimmerman and his family say the cries are his. Listening to the barely audible screams, I find it difficult to believe that anyone could say for sure who it is.

In any case, based on witnesses' reports and injuries that were treated by paramedics and noted by police (which included a bloody nose and cuts on the back of Zimmerman's head), Zimmerman shot Martin in the course of a violent struggle—i.e., in the heat of the moment, not in cold blood. He may well have overreacted, but if so his crime would not be tantamount to deliberately running over a black man with a pickup truck just for kicks, as Morris Dees of the Southern Poverty Law Center has suggested.

Is Florida's "stand your ground" law the reason Zimmerman has not been arrested? Zimmerman claims Martin knocked him down and repeatedly smacked his head against the pavement. He says his gun was exposed during the scuffle and he feared Martin would grab it, so he drew the pistol and fired. In these circumstances, a right to "stand your ground" would make no difference, since there would be no feasible way to safely escape.

Under Florida law, the crucial question is whether Zimmerman reasonably believed deadly force was necessary to prevent Martin from killing or seriously injuring him. The same defense would have been available to him under the "duty to retreat" standard that applied in public places prior to 2005—even if Zimmerman threw the first punch.

Another change to the law seems more relevant: To arrest Zimmerman, police need "probable cause" to conclude not simply that he killed someone but also that his use of force was unlawful. Yet probable cause is the same test that applies to all other crimes, and if it has not been met so far the fault probably lies in an incomplete investigation, rather than the requirement that police have a "reasonable belief" a suspect broke the law before they charge him.

Showing probable cause, of course, is not the same as proving guilt beyond a reasonable doubt—something to keep in mind if, as now seems likely, George Zimmerman's claims are scrutinized by a jury.

Jacob Sullum is a senior editor at Reason and a nationally syndicated columnist. Follow him on Twitter.

© Copyright 2012 by Creators Syndicate Inc.