A man serving 20 years to life for murder will get a new trial because the judge failed to tell jurors that a robbery victim is justified in using deadly force against an assailant who is not presenting a lethal threat.

The Appellate Division, First Department, cited various errors in overturning Kelly McTiernan’s 2010 conviction to second-degree murder. But the key finding in a decision handed down Thursday dealt with the standard for justifiable homicide, and how that standard differs for assault and robbery victims.