Does strengthening self-defense law deter crime or escalate violence?

An interesting and important question. It is a question looked at in a new NBER working paper, Does Strengthening Self-Defense Law Deter Crime or Escalate Violence? Evidence from Castle Doctrine by Cheng Cheng and Mark Hoekstra. The abstract reads:

Since Florida adopted the first castle doctrine law in 2005, more than 20 other states have passed similar self-defense laws that justify the use of deadly force in a wider set of circumstances. Elements of these laws include removing the duty to retreat in places outside of one’s home, adding a presumption of reasonable belief of imminent harm necessitating a lethal response, and removing civil liability for those acting under the law. This paper examines whether aiding self-defense in this way deters crime or, alternatively, escalates violence. To do so, we apply a difference-in-differences research design by exploiting the within-state variation in law adoption. We find no evidence of deterrence; burglary, robbery, and aggravated assault are unaffected by the laws. On the other hand, we find that murder and non-negligent manslaughter are increased by 7 to 9 percent. This could represent either increased use of lethal force in self-defense situations, or the escalation of violence in otherwise non-lethal situations. Regardless, the results indicate that a primary consequence of strengthening self-defense law is increased homicide.

One thing that is interesting is that they don't seem able to deal with the issue as to whether the increase in deaths is due to an "increased use of lethal force in self-defense situations, or the escalation of violence in otherwise non-lethal situations". I take it this to mean that they can't tell whether more attackers are being killed by would-be victims or if the level of violence is being forced up in general. And I would think that for many people that difference is important. In the first case fewer victims are being killed while in the second more victims are dying.