Utilitarianism/consequentialism is a metaethic, so it's a way of deciding what to do with a value system rather than a value system in itself - the paperclipper is a utilitarian even though it values paperclips rather than people.

You're correct that the original post makes assumptions about what the reader values. I think that's often worth it for efficient communication, though - the only alternatives I can think of are speaking in general or abstract terms ("a really bad thing happens", without being able to give an example like "a person dies"), or stating the assumptions.

I think gun control probably is privileging the hypothesis, according to most peoples' stated goals - they think gun control matters because it's related to safety, and they value safety, even though there are dangers more common and easier to control than guns. (I don't know off the top of my head what the low hanging fruit is for safety in first world countries, but transportation and preventative healthcare seem like possible candidates.) How close their stated goals are to their actual goals is a different question.