We want jurors to do this, to not only apply the law but also to use layman's common sense. What concerns Brown is the lack of instructions judges give juries about how to interpret the law in light of moral and social norms. It's well known that judges have to interpret statutes, but, in all but the simplest of cases, the statutes don't have clear meanings. Jurors don't apply the law by rote. Brown, who believes further studies are warranted in jury interpretation of the law, recommends that judges redesign jury instructions to help jurors become more competent at interpreting legal language.
Jurors have to interpret the immediacy of the threat.
Often, the spouses that they kill are not coming at them at that moment with a knife. There's usually a long pattern of serious abuse. These are not simple, straightforward cases.