I would like to suggest something to put a lot of this into perspective and maybe make some peace between the two sides here.
There is a theory in psychology known as Maslow’s hierarchy of needs. It is typically represented by a pyramid with the lowest level being physical needs and the highest level being self actualization and the middle everything in between. Very basically, it says that a person has various levels of needs, and if a more basic need is not satisfied, a person will not even have the urge for something higher up on the pyramid. Thus, for example, if a person hasn’t eaten in three days, he won’t be feeling the urge to belong, etc.
If I were to ask, what is the most meaningful, the top of the pyramid or the bottom? Most people would say, of course, in truth the top is much more meaningful. Yet the top means nothing to a person who doesn’t have the bottom.
The Rambam explains in a few places that the purpose of the peace and all the good promised throughout Tanach is to one day create an environment for which man will be able to perfect himself and become close to Hashem. Similarly, all the rules of Bein Adam L’chaveiro are to create a functioning society. And the point of a functioning society? So that a person has the capability to work on perfecting himself. The functioning society is not the end, it is the means.
Looking back at Maslow’s pyramid in light of this understanding of mitzvos, one might say that in the world of the Torah Shabbos may very well be at the top of the pyramid. It is perhaps the ultimate demonstration of one’s belief in Hashem. Perhaps from a detached perspective it is more meaningful than anything else. But one cannot deny that murder is at the very bottom of the pyramid. No society can sustain itself if people can get away with murder. Therefore, practically speaking, we need to worry about that first. And until people are not deficient in hilchos bein adam l’chaveiro, we should hold off telling them off about Shabbos, because for them, since they are still closer to the bottom, not killing truly does have more meaning.
Anyway, those are my thoughts.