In the spirit of the OP (I think?)…instead of comparing the murder of a homeless man to the murder of John Lennon, we could as easily ask about the moral difference between killing Martin Luther King or Adolf Hitler at the height of their respective careers, assuming opportunity and means.
a) A general social contract principle of the anti-vigilante sort says it would be equally wrong, and very wrong, to undertake either action as an individual, rather than leaving it up to a court; whereas (depending on how you feel about capital punishment) it might be entirely right and perhaps our duty, collectively, to kill Hitler if we had opportunity and means to have arrested him and brought him to trial for his deeds.
b) The contrary prevailing principle (I’ll call it the “Dirty Harry” principle) discards the anti-vigilante position on the grounds that opportunity doesn’t always present itself and severe evil must be stopped when opportunity and means are available. Some would even say that each individual has the moral responsibility to act rather than to defer to the collectivity unless representatives of that collectivity are present and able to act. Once you’ve taken that position, you’ve set yourself up to say that you might under certain circumstances kill someone for reasons other than self-defense, and that questions of who, when, and why will involve evaluating and placing a relative value on the person. For most of us, that would not easily lead to the conclusion that killing a homeless man (or, for that matter, John Lennon) is in order, although doing in Adolf Hitler certainly might be.
c) A less prevailing, less common principle that would permit individuals to kill according to their own discrimination and claim moral righteousness in doing so is the creepy self-righteousness of the self-defined Normal, who tend to believe that they are in the historical majority and, to a lesser and threatened extend, in the current majority, but that society and culture and all that is good in the world is threatened by non-Normal people, who are easily recognized by the Normal, all of whom in their hearts know the non-Normal are wrong and evil in their entire way of being here, and that their elimination is necessary. Not everyone with such belief systems also tends towards taking individual initiative in ridding the world of non-Normal people by killing them personally–most trust to the social mechanisms of criminal justice, mental hygiene, economic, and other such systems to eliminate them from circulation, and give their little pushes to those systems and their systematic behavior–but others do, and when they do, Adolf Hitler is again the order of the day, if you catch my drift.
