(albeit thanks to human nature not very easy to apply)
Treat people the way you want to be treated.
If somebody is doing something and no one is harmed, mind your own business.
If somebody is doing something that harms a third party without the third party's consent, do something about it.
If somebody is doing something that harms only themselves, be very careful how you proceed.
(For the record, the terms I use above I define this way: Somebody is one or more individuals, harm is something that uncorrected will have a permanent negative effect on a person's life.)