I read your last paragraph 5 times now and still can't make sense of it.

One should drink water if one wants satisfy one's thirst. Here should is loosely used to mean that it is the optimal instrumental action to reach one's terminal goal. One should not kill is however a psychological projection of one's utility function. Here should means that one doesn't want others to engage in killing. The term should is ambiguous and vague, that's all there is to it, that's the whole problem.