But what does "sin" mean if anyone can decide that some behavior is a a sin? And others aren't?
Where's God when you need It?
Personally, I think some things are pretty self-evident. Doing harm to other, showing disregard for shared spaces, destruction without regard for consequences, etc., are not needing a booming voice from heaven to tell you its bad. Experiencing it tells you that.
I don't think dumping toxic waste into the local river is a sin. The Bible doesn't say anything about toxic waste.
It says in the first chapters of the Bible that we are to be responsible stewards of the earth. Furthermore, Levitical law clearly teaches that the land is God's, and that is why they had things like the Sabbath year, among other things, which was for the purpose of the land not being continually exploited for gain, and that the poor can have that harvest. Even while they had a system of land ownership, ultimately is was God's, not theirs. That is Biblical. This all is about respecting the land.
Secondly, it's common sense. Water flows downstream where others livelihoods depend. By putting toxic waste upstream of them, you are in fact sinning. You are polluting and destroying the natural environment because of greed, or laziness and no respect of others. No booming voice from heaven necessary to understand that either. That is sin.
For some people however, they need an authority like the Pope to spell this out for them, what should otherwise be naturally obvious.