As an atheist don't you really feel fear for committing sins which are not violating national laws?

That mentality is still around today: anti-abortion activists resort to terror – murder and bombing – in their holy war against abortion. To them, it doesn’t matter if it is illegal; and even if they think of it as immoral, they argue it is for the greater good and the lesser of two evils, and thus not a sin.

Since you seem a bit confused about atheism, let me explain with a Venn diagram:

For Christians, the Sin circle includes for instance murder, lying, hot gay sex, and infidelity.

Will my parents go to hell if I don't wear hijab, they tried to convince me and they provided it to me but I don't want to wear it?

The law in most country defines murder as illegal, but not always lying (some exceptions like libel or perjury exists, but generally lying is actually not illegal). Hot gay sex may be illegal in some countries, but not in others. And in most Western countries, infidelity may be immoral and in religious contexts a sin, and maybe even cause for a divorce, but it is not illegal per se.

Well, we don’t care if an act is considered a sin. We care if an act is considered immoral or illegal. “Sin” is basically Someone Else’s Problem™ to us. If you really don’t want to have hot gay sex because you think it is a sin, then don’t have hot gay sex. But don’t force your view on anyone else, because then you are the problem.

For most of us, the Immoral circle includes murder, lying and infidelity, but not necessarily hot gay sex. Religious people may think hot gay sex is immoral because it is a sin, in turn because their holy book says so, but then it falls more under Sin rather than Immoral. No matter how religious people try to make hot gay sex to be immoral, it always comes down to violating divine law, i.e. sin. If you’re not religious, hot gay sex doesn’t harm anyone and mostly done between consenting loving adults – it may not be your thing, but love can hardly be seen as immoral unless you bring religion into it.

Why do people hate Nickelback so much? What makes them different from other popular bands like Linkin Park, Green Day, etc.? Is it just because they're Canadian or what?

Hello, Chess 365 !

So where do atheists come into this?

As you can see, some acts may overlap in some ways. Infidelity fits as both sin and immoral, but it is still not illegal. If hot gay sex is illegal, then you have an overlap with sin. And murder usually fits all three.

Why do old men think young women and girls would want them over guys their own age?

And before you ask, there are a few things which does not fit “sin” but does fit “illegal” and “immoral”; holy war is one of them, and the US fought a civil war about whether slavery should be one – one side argued that slavery was not just not a sin, but a virtue of a god-fearing society and should be legal everywhere in the United States. Luckily, the other side won.