what is ethics anyway?

"don't do to the others what you wouldn't like to be done to yourself" was the first "rule" i've learned in my ethics class. and then it came the "mother's paradigm", "don't do to the others what you wouldn't like to be done to your mother". or something like this, whatever. anyway, this is stupid, i said in the class, much for the astonishment of either the teacher and the rest of the students. if i am walking down the street and see a friend of mine being robbed, i may call for help or even go there to beat the thief. if i see the same thief robbing an unknown, i won't risk my life; i will call the police instead. i was branded as selfish after this statement. but truly, who wants to be a hero nowadays?