A person is intelligent, a mob usually not. If everyone is rational and did everything to his best interest then you get chaos. Same if everyone did everything that benefits the environment. The idea of an utopian society is just an idea. It would not be reality.
thus the golden rule applies here; do unto others as others to unto you.
in the old jewish law, there is NEVER such a golden rule, just god's word and obey, smiting and killing. It is only until Jesus' time when he preached about this.
this golden rule has been part of buddhism for ages and many have suspected that jesus went to india to understudy the philosophy of buddhism. That is why jesus' teaching is so similar to buddhism.