Making it easier to kill (access to a firearm) doesn't necessarily lowers the cost of killing. The cost of killing lies on the consequence of killing.

Some people seem to be blind to the fact that less penalty and defunded/broken police and justice system lowers the cost of killing (by making it hard to bring murder to justice and the due penalty) https://www.nytimes.com/2016/03/16/world/europe/anders-breiv...