I interpret the question as meaning what ethically should you do, moment by moment. Let's sneak up on the problem.
What is the purpose of an animal (say a mouse in the wilderness)? It has a natural design which has been selected to keep it alive until it can successfully reproduce. That is all there is to it - no higher purpose can be detected. If the mouse had a choice, which it doesn't, we would advise it to act according to its nature, which is to do those things which maximally support its ability to be an ancestor.
What is the purpose of a human being? Like the mouse, we are constructed to a design which optimises our ability to be ancestors. However, we are social creatures, and, we cannot survive and be reproductively successful except in social contexts created by the social groups to which we are affiliated. These groups assign roles to us, and within these groups we are obligated to participate in role-negotiation, objective setting, planning and execution in a way which furthers the overall interests of the group and our own role within it.
This behavioural framework of social interdependence was presumably selected for when we lived in extended kin groups, and subsequently as we lived in social groups where we were non-related. Robert Trivers' theory of "reciprocal altruism" has a lot more on this, in "Natural Selection and Social Theory: Selected Papers of Robert L. Trivers" (2002).
Evolutionary theory doesn't have a problem if my social group wipes out your social group (this was the whole point of Trivers' analysis), so applying an unmediated universalistic ethic can't be right: a critique of pacifism. Within the group, however, skillful diplomacy and the ability to build strong consensus are clearly pro-survival. The earliest religions seem to have been programs for cementation of social bonds within social groups riven by discord. These religions were given a universalistic spin (e.g. Christianity, Buddhism, Islam) when they became the property of empires which needed legitimacy. Note that for these religions (possibly excepting Buddhism), universality stopped at the boundaries of the empire, where the pagan or infidel was encountered, and slaughtered.
My personal conclusion is that we should do those things which are in conformity with our natural design. Namely, act to maximise our abilities as human beings to live and work effectively together as one community, or an alliance of communities. Because we are inter-dependent, this is the best outcome for all our long-term interests, when it can be made to work. But sometimes you have to choose which group to align with, and then whack the other guy. However you call it, you will then just have to live or die with the consequences. The best idea is probably to choose to support those groups which (in your opinion) hold out the best hope for long-term human social progress - where you can identify which that is!
When I was doing AI (Artificial Intelligence) and involved in agent theory, I was struck by the following foundational issue: researchers usually decided arbitrarily which problem their system was going to address. It might be successfully stacking bricks on a table, playing chess, or constructing scene description from primary visual sensor data. Obviously the problem selected had a very considerable effect on the kind of systems and solutions generated. To get away from this inherently arbitrary problem-selection process, some researchers cycled back to considering homeostatic systems by analogy with biological systems, where the motivation is to survive (so as to have descendants). This was in fact the shape of the pre-AI research programmes in the golden age of cybernetics (e.g. Introduction to Cybernetics, W. Ross Ashby, 1956 - one of my early heroes).
Most biological systems are set endless tasks by the environment. By contrast, some humans have the luxury of being quite well-off, and have no immediate survival problems to address. How should they spend their time when all courses of action appear to be pointless? Wealthy and dysfunctional film stars often provide examples for analysis, as well as the recent angst expressed by the remaining dot-com billionaires as to how much to leave in their Wills to their offspring.
I am struck by how many people invest their lives in their enthusiasms (what we used to call "hobbies"). I am not the first person to observe that all enthusiasms seem slightly odd to those who don't endorse them. In the best of cases, people can make their lives into an art-form through the level of skill and accomplishment they develop in their special areas of interest. This implies that culturally-enriching the common human condition is as much a purpose of life as the more mundane team-activities which over the millennia kept our ancestors physically alive. What is less aligned with our underlying nature are such opt-outs as suicide, or the social-suicide of withdrawal from all social activity, where this cannot be placed in a broader social context. Crudely, to do these things pointlessly is, in the microeconomics jargon, shirking.
