Alright, so last night I had a dream about someone being murdered in my apartment, basically the floor right above me, and there's this guy who always plays loud music and throws stuff out his window (I'm guessing he's distressed), what would this mean?
Dreams don't always make sense, nor do they always have to make sense, although founder of the psychoanalytic field Sigmund Freud believed in dream interpretation: that dreams have a manifest content (literally what goes on in the dream) and a latent content (symbolic, not-initially-apparent meaning of the dream). It's worth noting there's no scientific evidence to back this up, and it's just something Freud believed.
In the case of your dream, it could simply be that your mind associated your neighbor's aggression/distress with a display of aggression (homicide) and somehow connected the two in one dream. Freud would probably argue something along the lines of "Your repressed anger or frustration towards this individual manifests itself in the form of a dream in which he is murdered". Don't overthink this, and don't think your dream means anything bad. Dreams can be pretty bizarre.
There's no external meaning to a dream. It's the dreamer who makes sense of it, so they don't have a general meaning that could apply to everyone and they can't be "interpreted".
That been said, there are hints to find our own sense out of a dream: try to see it as it is speaking all about you (it is! as you are the dreamer). The house is you, the various floors are different aspects or levels of your personality (maybe mind, body, emotions... but that's just an example) or of your life (work, family, leisure... as another example), and the people involved just various aspects of your own personality, we can all see "different me's" inside ourselves (we are not the same "personality" when applying for a work than when we play with our kids... etc.)
So, and just as an example: maybe you have one of those auto-critical minds that never shuts up (the noisy distressed neighbour upstairs) that "kills" (murders) your spontaneity (another part of our personalities: the playful child we all also are). Or maybe vice-versa: you are very impulsive and the dream reflects your ignored fear of the consequences (that kills the much needed grown up we must also be).
Maybe these examples can help you gain an idea of a way to interpret your own dreams. After all, it's our own brain that produces images to balance our mind, so usually they reflect things that when we are awake we don't take into account or don't pay enough attention. In my examples, the "advice" that can be applied could be "don't be so hard on yourself, as you are "killing" your joy, spontaneity... etc" or "be more responsible, as you are "killing" your health or your future... etc".
You can also find meaning in the emotion itself. Seems you are kind of an spectator in the dream, is your neighbour who is distressed and someone else is being killed. There are people that don't allow themselves to recognise they feel angry or they don't do something with their anger (anger is only destructive when it is not used to build), so they dream with things like "another one is distressed" "someone kills someone". Anger is much needed to survive and to live healthy. It tells us what hurts us and what things we want and don't want in our lives. So, if we "don't feel anger" we live sad and stressed lives. Anger is there but we don't listen and don't do anything constructive with it, so we dream with it. It's just like when we are thirsty, if we don't drink our brain constantly produces images and even sounds of water in our minds... isn't it? Well, dreaming is the same thing. In fact there's the saying: "I'm dreaming with a glass of water!" "I'm dreaming with my bed!" (when we are very tired) just because of that: don't forget and focus your attention on what you need.
In any case, it's only you who can tell you've come with the right meaning. You'll feel that something makes sense for you.