paralysis

[Angrboda]

Should by what standard? There is no 'should' for anything because there is no objective morality.

But as an evolved pack animal with instincts to band together with fellow members of my species, my sense of empathy and consciousness means I personally would prefer to look after the disabled person.

Exactly my point. In an atheistic worldview, there is no independent objective morality. There is no standard. There is no "should" of anything, as you mentioned.
One's own sense of empathy and consciousness (and I'm not saying an atheist can not "feel" empathy) does not entail any "shoulds" or standards on others. It's only your opinion, not binding to others in any way.

This is simply not true. We evolved to function as a group of individuals, not as solitary individuals, so certain group behaviors form a natural imperative for us. Morality of the group is one such behavior that while not possessing permanent, objective characteristics, does nonetheless provide for the setting of group norms that are binding on all members of the group, as a consequence of how the group treats rule breakers. There is morality, even if it is changeable and dependent on group behaviors. That doesn't make it any less real.

So if I was the disabled person in the OP scenario (and I know people in this situation), I would be very seriously and extremely threatened. Because its all relative, and "it depends", so each person I meet is an entirely open canvas of infinite possible moralities, because (in this world view) there is no independent standard objective morality. There is no "should" of anything, as you mentioned, and its all open for discussion.

That consensus morality tends to converge upon certain "absolutes" is evidence against this "blank canvas of morality" view that you are presenting. Morals are partly inbred and partly taught. The institutions of family, culture, and government serve to perpetuate a system of morals and stabilize it against the possibility of radical change. There are shoulds just as if the morality were objective. That it is a consequence of group behaviors that are inbred in each of us gives it the substantiality that it needs. This is not to say that a society couldn't change its morals such that killing your disabled person is moral, but that it does not align with recent societies and is thus de facto immoral.

Part of this consensus morality includes valuing others as human beings. You don't get to wipe that away because you have a philosophical objection. It's a very real part of the fabric of our morals, even if you don't see the reasons for it being so.

But for a standard, independent, outside source of morality (that is, objective morality), no body would even begin to imagine pondering on this, and the issue is absolutely and completely out of question. And because of that, if I was the disabled person in the OP scenario, I would feel completely safe and relaxed. Which also eventually benefits everybody.

That depends on the objective morals in question. Even if there are objective morals, you're simply interpreting the scenario as falling under them. Remember that the bible didn't proscribe either rape or slavery. This is an example of people following 'objective' morals that we today would consider unthinkable. These two issues may well lie outside the bounds of any objective morals and be a mere human invention. What evidence do you have that killing the disabled person is covered by an objective ethic, as opposed to being merely dragged along on the boundary of other morals that truly are objective?