Depending on which state he's located in would determine if he needed to "try and retreat"...he should have waited til they were inside his home.....THEN, in a state like NY, he could have argued that they backed him into a corner or he otherwise had nowhere to retreat...or if in a Castle Doctrine state, could have legally fired his "warning"....center mass.
I think it would be interesting if his lawyer brought a tape of Joe Biden into court as part of the defense...of course after alerting all of the media. Likely only Fox would cover it.