All knowledge is ultimately based on assumptions, and those assumptions ultimately hang in thin air. We rely on our senses to observe the world around us, and we rely on our mental faculties to interpenetrate what we observe. But we have no way of knowing if our senses, and our mental faculties, can be trusted, if they are accurate to some objective reality.
It might very well be that what we take in from our surroundings and environment isn't an "objective reality", but an incredibly twisted and mangled version which is has more falsehoods, inaccuracies and errors, than any truths. To say that the sky is blue is an incredibly "human" observation, which relies on our eyes having divided the light spectrum into arbitrary ranges we call colors. There is no grounding to distinct colors, other than what our eyes and brain arbitrarily imposes to make sense of what it's taking in. It's a clever, but arbitrary, way to look at the light spectrum.
But we do the best with what we have, there is no point in wallowing in pity, we simply work with the tools we are given. Now, even if we cannot justify it, it does appear to us that murder is morally wrong. We are repulsed by it, we wish to avoid it, we want to minimize it, we label those who do it as being flawed and broken, and we have all sorts of theories and ideas about how murder doesn't fit in with how human life operates.
Even though I cannot point you to any objective anchoring as to why murder is wrong, I can neither point you to any objective anchoring for anything. I cannot prove that what I see, touch, feel, smell and hear is really there, nor that what I am observing is in any way accurate. Even the most basic mathematical facts are suspect, if you can't verify the mental faculties behind it.
So why reject the moral facts I cannot verify, but accept the observable, evidential, mathematical facts that I cannot verify? Who gets to choose which ones gets a pass? I think murder is wrong for the same reason I think the sky is blue: because that's what my senses, body and mind tells me.