Fifty's going to have a lot of catching up to do when he looks at this thread.
Humans vs nonhumans: I assume you aren't talking about ETs but animals and plants. Animals and plants don't appear to have actionable systems of ethics, at least not insofar as they relate to human standards of ethics, which if both exist, would be inherently different based on motivations.
A newborn baby, or a person in a coma, doesn't have an actionable system of ethics - but people who think that humans
qua humans have special moral value still think that newborn babies and people in comas count.
Personally I don't think that there is any good reason to make a hard and fast moral distinction between human and non-human life. Of course, most human beings are (probably) in a different moral category from most non-humans, but that is surely because of characteristics that they happen to have that are usually, but not necessarily, associated with being human - it is not because of their humanity itself.
Maintaining the life of a violent offender is tantamount to promoting violence.
Sorry, I can't see that at all. You might as well say that failing to execute a thief is tantamount to promoting theft!
Failing to punish a violent offender is tantamount to promoting violence; but one may maintain the life of a violent offender and yet still punish him or her. I suppose that the question really revolves around how you define "pacifist". If a pacifist is someone who opposes war under all circumstances, then there's no reason why a pacifist couldn't support capital punishment, because that's a different issue. If, however, a pacifist is someone who opposes the use of violence, then I can hardly see how a pacifist could support capital punishment. The pacifist (on this definition) presumably seeks to minimise violence; the violence involved in incarcerating someone against their will is presumably less than the violence involved in killing someone (or releasing them to commit more crimes); so it seems that the only consistent view for a pacifist (on this definition) is to oppose capital punishment. Unless, of course, they thought that executing prisoners would have such an effect on society that violence would be reduced overall (because of the deterrent effect, or something). To reason like that, they would need to be some kind of consequentialist, holding that a violent act that has overall consequences of reducing violence is to be preferred to a non-violent act whose overall consequences increase violence. However, a deontologist pacifist - who thinks that violent acts are intrinsically immoral no matter what their consequences - would reject that reasoning.
Of course, that's for a pacifist who is a sort of fundamentalist about pacifism, and who think that minimising violence must take priority over all other considerations. I can imagine a pacifist who wishes to minimise violence, but who also thinks that other considerations may override this under certain circumstances. Such a pacifist might think that if life imprisonment is considerably more expensive than capital punishment then capital punishment might be justified on those grounds, with financial considerations outweighing pacifist ones. So from that point of view, a pacifist might favour capital punishment, but on grounds other than pacifism.