If no one follows any rules and morals, why should I? (see the details)

As everyone else, I’ve always been thought about “good” and “bad”. I’ve been thought the rules, principles, morals. “This is good and people that do this are good; this is bad and people that do this are bad”.

However, the more I think about “good” and “bad” deeds, I notice something:

a) everybody does the bad anyways. Everyone lies, cheats, bribes, breaks the law, uses people, even kills to achieve what they want. There is no soul on this earth that hasn’t sinned in one way or another at least once.

b) our notion of what is good or bad is based solely on the context: who did it, why, when and how. When you kill your girlfriend out of jealousy, you’re a monster. When you kill the enemy soldier in war, you’re a hero. When you gamble, it’s illegal and bad; banks do that all the time and no one bats an eye. Being gay was a sin centuries ago, but is acceptable now. All in all, the deed never changes in it’s core in any way, it’s neither good nor bad by itself; it’s condemned or justified solely on the context and circumstances.

c) if no one knows you’ve done bad, it’s like you haven’t done it. Really, how can anyone react on what you’ve done if no one knows you’ve done it? If you kill someone and no one knows it, you never face consequences. How can we say you’ve done wrong if we don’t know that, and if you never took punishment for what you’ve done? In other words, if you’ve done wrong and no one knows it, have you really done anything wrong?

Which brings me to:
If good and bad depend on the context, how can we say something is good and bad in the first place?

If everyone does bad anyways, is it really bad?

If there are rules, but everyone not only consciously break them and don’t fear any punishment, but don’t face any punishment either and even get benefit from what they did, what’s the point in following any rules and principles anyways?