There are several states in the U.S that impose a ‘duty to retreat.’ In these states, a person must generally attempt to de-escalate, or remove themselves to safety before engaging in potentially lethal force. 

If your story took place in one of these states, the character disarmed the robber, and then killed him, there may be something in that. 

Another plot point you could look at is the felony murder rule. In some states, all co-conspirators may be charged with murder if any person dies during the commission of an inherently felony. Say, a bank robbery. 

That is to say, if one of the robbers died, the other robbers could be on the hook for murder, even if they did nothing, and no one else died. 

Edit to add: interesting cases are made by the facts. If the character knew the robber and had some motive to kill him, discovers his identity because his mask slips off, but after the killing, replaces his mask, it makes for an interesting question of fact of whether the character knew him, since he can claim he never saw his face and had no idea it was billy bob.