A relativist is someone who doesn't believe in absolutes. They don't believe anything is objectively wrong Most people sit somewhere in between relativism and absolutism; believing some things are absolutes and others are relative. For example, as you correctly point out, whether murder is 'wrong' can be relative. However, some things, such as female circumcision, are just plain wrong. Now culture etc. can be used to 'explain' why these things are done, but it doesn't mean they are ok. I would be pretty confident in saying that FGM (Female genital mutilation) is one of those absolutes. Some things are just inherently wrong, and as such, one can objectively say that it is wrong. Objectivism is single- minded and uncompromising in nature, but as I've stated, somethings are just wrong.
As I said, there's a continuum- some people are mostly relvatist some people are mostly objectivist, I think you'd be hard pressed to find someone who is a complete relativist and who can justify that position, but some ethicists do.
I don't think what you wrote is nonsense at all. It's good to explore ideas and consider different perspectives. I don't view human life in the way you do, i don't think i could say, 'this person should live and this person should die because i deem this person to be of more value to society'. I don't think any person is capable of making that judgement.
I agree, of course we can't get involved in every cause.