Q&A 6, Second Answer

My question is: Is there a point at which a human who, due to circumstances beyond their control (such as medical issues or living situation) causes a great amount of harm to other sentient creatures, should disregard their own health and/or life in order to cease causing that harm

I think that there is such a point.  However, I think that point is so extreme that no such situation would be at all likely to arise.  Some lesser situations of 'necessary evil' do arise, but I think that those cases do not approach the point mentioned above.

For example, if a person cannot cease eating meat and other animal products for reasons of health (as is the case for a few specific people), that person has the right to continue living.  They should keep searching for alternative ways to meet their nutritional needs, but if they cannot do so they are not obligated to end their own lives by malnourishing themselves.  I think that they have the right to do this for a number of reasons, first and foremost of which is that people have a right to self-preservation.  Also significant is the fact that many (if not all) non-humans are lower on the sliding scale of sentience than are humans, and as such possess lesser moral value.

If, on the other hand, a person had (for example) to eat the brain of three dolphins dolphin every single day in order to survive, that person might be beyond the point I mentioned above.  They might be obligated to die in order to cease causing the deaths of multiple highly sentient creatures on an ongoing basis.  I am less certain that others would be justified in forcing them to make this decision - I will have to think further on that.  Luckily, however, no such ailment exists to my knowledge.

Q&A 6, First Answer

My question is: What variables in sentient beings are morally relevant?

Quite a lot, I think.  I am not necessarily qualified to provide a comprehensive list, certainly not without quite a lot of thought and debate with fellow ethics enthusiasts.  That said, I think certain variables are especially important, and (perhaps as a result of this) are also more readily apparent.  I can think of two in particular which stand out.

The first is the ability to feel pain, both physical and, in higher life forms, psychological.  The more acutely and complexly a creature is able to feel pain, the more ethically problematic hurting or killing that creature becomes.  Thus a shellfish, in spite of its biological status as an animal, has little to no moral value based on this variable.  It has the same moral value as a plant, in fact.  A mouse has far greater moral value, due to its ability to feel physical pain and psychological pain and panic.  A human has still greater value, as it can feel all the same things that the mouse can, but with far more complexity.

The second is the ability to reflect on one's own existence.  This one is trickier, as it is difficult to determine to exactly what extent various clearly sentient creatures are self-aware.  Regardless of its detectability, however, I think that self-awareness is a relevant component of moral value - if one can reflect on the value of one's own life, then one can place value upon it independent of what it may possess upon its own.