Another tidbit from the ancient weapons expert I saw in the documentary, he said that cutting off an ear was done to mark someone who had betrayed/grossly insulted someone. So, Peter wasn’t trying to kill the man, but merely to mark him as a traitor. It was an act of vengence not of attempted murder. Apparently, Peter knew that man–and for some reason felt he deserved the blow more than the others with him. John seems to have known the man as well because he was familiar with some of those who worked in the High Priest’s household. So, it probably wasn’t a rash, random act that Peter did, but aimed at a particular person for a particular reason. By healing the man Jesus showed that we are not to exact revenge/vengence in his name.
This doesn’t mean, of course, that people can’t be held accountable for crimes, rather we are not to take personal actions against others we believe have wronged us. All this to say, that if Peter struck from the back, he’d have done it with his right hand. But, if Peter wanted to make a statement, he would probably have struck from the front. It could be the man turned to avoid the blow and so his right ear was struck as Peter came at him from the front. The scriptural account doesn’t go into all the details. The Gospel writer had no need to explain the situation to his readers, who would have been familiar with the cultural setting and the weapons involved. He merely wanted to show that Our Lord had no intention of resisting and that he, even in that moment, wanted to teach us to love our enemies.