The question is easily understood in common conversation because you share the societal moral system.
The point is not that you cannot define "should". It is exactly the opposite. You do, so quickly and easily that you don't have to think about it. You are intimately familiar with your own system of morality.
Telling science to define that system for you, meanwhile, is nonsensical. Moral value is not objective. All moral codes are, at the most basic level, arbitrary, as are all statements of worth. No particle of goodness or beauty exists. The Mona Lisa is no more objectively beautiful than a snuff film, and Mother Theresa was no more objectively good than Hitler.
All of these things are opinions. The system you use to determine those opinions might be common in your society or unique to you, but it is still, at its most basic level, arbitrary; "murder is wrong" means nothing unless "wrong" has been defined, which requires that you operate within a moral system that defines it.
Once the system is in place, science can answer the question trivially easily, but you still have to define your terms. In the same way, you coukd ask "is the sun big?" and get the rejoinder "what's big?", because "big" is arbitrary. Is a cell big? Compared to an atom, certainly, but not to a building.
You want a scientific answer to "is murder wrong?" Then all you have to do is define "wrong".
Which, like I said, isn't hard.