Some robbers don't understand that what they're really doing is setting up a wager. The stakes are not the money or property in question, but the lives and safety of both parties. Robbers think they have the upper hand, because usually they are armed, and hence, the victims are unwilling to wager their actual lives to protect their property.
But when you take that extra violent step, you remove the premise that being complacent will keep you safe. And for some people, that's the trigger to fight back, because the game has changed, and now the stakes are clearer.
What makes the average robber faulty is they don't understand they're part of the gamble, so the notion of losing or what will be lost never occurs to them. And while it may sound like I'm being empathetic, I absolutely am not. They are absolute pieces of shit for putting lives at risk (including their own) over a few dollars, and they absolutely deserve it when they lose after going all in.