Given that you are definitely NOT going to heaven, being non believers and all, and that you are probably not going to hell because there is no afterlife. What determines your actions? Do you have an attitude or set of guidelines in place of the rigid rules placed on us by religions. For myself, I am guided by the principle, "Do no harm" , re judging others, it is similar, as long as no-one and nothing is adversely affected, consenting adults can do what they like.
Some things are no brainers but in general I ask myself 3 questions before doing something questionable. Particularly if my feelings are engaged.
Will it cause any harm?
Is it self serving?
Will it be beneficial?
If either of the first two are a yes then the last is a "probably not" and needs further reflection.
Although I often fail to live up to my ideals, I try to guide what I do and say in accord to the principle of the good, in a general sense. Among the goods I consider important are love, creativity, pleasure (physical and mental/emotional), reverence for life, and a concern for the feelings of other people.
I have come to believe in Karma as I understand it to mean giving out what you wish to receive. I have read different versions of Karma. Basically I believe in treating others as I wish to be treated; but not guarantee that this will happen. I believe this is a kind of energy we emit from our spirited beings.
To reduce my share of being a problem (goes beyond the carbon footprint). And to be proactive in preservation and awareness to the serious issues life on this planet faces (overpopulation is the 'core' issue).
Without a doubt overpopulation is the biggest issue and the one politicians and business are refusing to accept.
There is a quote that says something about one's character when no one is watching or something like that. For me, my primary compass is the mirror. I believe, rightly or wrongly, that we are born with a moral compass and we all have a choice, with every action and inaction, words, etc. to do the the right thing or the wrong thing.