Was your prior assumption that forensic evidence must exist in every case—and that if it doesn’t, then there’s no way to convince a jury of someone’s guilt?
As in, as long as I clean up really well afterward, I can pretty much do what I want?
I think you're missing the slippery slope that this goes down. The criminal charges were way too low, given the alleged actions. The state admitted it had absolutely no forensic evidence. The judge was perfectly fine with this and selecting a jury that was OK convicting in this circumstance. This pretty quickly pretty us down a path of "you're guilty of at least one crime since you've been indicted, maybe a more serious one if we have some evidence".
The standard of both investigations and the effectiveness of mostly part time police were pretty low. Most police were elected or appointed officials with deputized staff. The levels of training and professionalism were poor.
Why do you think many places would execute petty thieves? So few were caught generally speaking the punishment had to be severe.
The case was a shooting. It seemed remarkable they had neither a gun, spent cartridge cases, blood, flesh, wounds, or anything in the way of physical evidence.
Evidence, not physical evidence. The instructions seem entirely reasonable.
There is no hard rule for physical evidence. If other evidence is enough to convince 12 jurors beyond a reasonable doubt, it could be very significant.
As in, as long as I clean up really well afterward, I can pretty much do what I want?