I am an emotivist and do not believe anything is good or bad in an objective sense. I think some Indians may have had guns by the 1700s, but their bows and arrows weren't terribly outclassed by many of the old muskets back then either (I'm actually discussing that at my blog right now). The biggest advantage of the colonists was their ever-increasing numbers (while disease steadily drained those of the natives). The indians frequently did respond in kind to killings and the extent to which they could do so would strike me as as the most significant factor to take into consideration when it comes to the decision to kill them.
There is also the factor of trade relations that could be disrupted, but most people engaged in prolonged voluntary trade are going to have significant ass-kicking ability or otherwise they would have been conquered and their goods seized by force already. I understand Peter Leeson has a paper "Trading with bandits" disputing that point, but the frequency with which dominance based resource extraction occurs makes me think the phenomena he discusses only occur under very limited conditions.
As an emotivist, you might be interested in reading After Virtue, particularly the first three or four chapters. He presents a rather compelling argument against emotivism, and if you want to maintain your emotivism you probably ought to find some rationalization defending yourself from his argument.
For a long time, I wanted to ask something. I was just thinking about it again when I saw that Alicorn has a post on a similar topic. So I decided to go ahead.
The question is: what is the difference between morally neutral stimulus responces and agony? What features must an animal, machine, program, alien, human fetus, molecule, or anime character have before you will say that if their utility meter is low, it needs to be raised. For example, if you wanted to know if lobsters suffer when they're cooked alive, what exactly are you asking?
On reflection, I'm actually asking two questions: what is a morally significant agent (MSA; is there an established term for this?) whose goals you would want to further; and having determined that, under what conditions would you consider it to be suffering, so that you would?
I think that an MSA would not be defined by one feature. So try to list several features, possibly assigning relative weights to each.
IIRC, I read a study that tried to determine if fish suffer by injecting them with toxins and observing whether their reactions are planned or entirely instinctive. (They found that there's a bit of planning among bony fish, but none among the cartilaginous.) I don't know why they had to actually hurt the fish, especially in a way that didn't leave much room for planning, if all they wanted to know was if the fish can plan. But that was their definition. You might also name introspection, remembering the pain after it's over...
This is the ultimate subjective question, so the only wrong answer is one that is never given. Speak, or be wrong. I will downvote any post you don't make.
BTW, I think the most important defining feature of an MSA is ability to kick people's asses. Very humanizing.