Alashuko The Fighter
I know the natural law and have heard people like Richard Dawkins admit there is no purpose for our existence. We exist. That's it. Enjoy your life. As an atheist, I would want to rid humanity. As a Christian, I would want to help people and find inner peace. As well as world peace.
Is your Christianity the only thing that stands before you and mass murder? If so, I can tell you two things: One, you need to be locked up; Two, then I guess just about every single atheist in this world is a better person than you.
You've created a false dichotomy for yourself. You say that being a Christian has made you charitable and helpful to your fellow man....and when you imagine how everyone else who isn't Christian lives, you deluded yourself into thinking it would have to be the most opposite of opposites (a mass murdering, vandalizing, raping and pillaging lost soul). Point to just about any non-religious atheist in your community (and if you live in the US, we're talking almost one person in five) and he or she would be proof that your idea is wrong. How would you explain an atheist helping in a charity? Repressed Judeo-Christianity? Or is it the more likely answer: You'd help people whether or not you were a Christian.
Your question, and the answer you pulled from it, are false to begin with. When you ask "Does an atheist have meaning", you are fishing for a "No" since you believe your religion gives the only "Yes". The correct answer is "
Mu", basically that your question is flawed and cannot be given a binary answer in the terms specified. Is there an overarching meaning to life? Probably not. Does a particular atheist have a meaning in life? Likely. Meanings for an atheist would not be handed down to above, but created themselves if they so choose. But Christians, and those of other religions, create meaning for themselves just the same.
It's like....Minecraft (bear with me): The game doesn't have any particular "goal" or "missions" if forces you to take. There is no hovering text saying you need to make a castle on a hill to please the almighty quest log...you are free to instead clear a forest and make a farm. You, however, are giving the impression that a Christian would load up the game and stare blankly at the screen until instructions would arrive, and pray for all the skeleton archers to be destroyed by His holy magnificence. And you have deluded yourself into thinking that an atheist, upon loading up the game, would immediately delete it from the hard drive, set the computer on fire, and kill his family. Neither is true in the slightest (and my proof is how well the game sold).
In short, you've muddied your ideas with made-up terms like "Natural Laws" and confused your conclusions with the idea of "True Purpose". Not once have you considered that if neither actually exists, that not a whole hell of a lot would change. If Christianity was proved to be a farce tomorrow, 75% of the US wouldn't go on a murder rampage....very very few really would even consider it. If there is no Heaven or Hell, few would chose to make their sole existence a living hell, or stand for someone making other people's lives that bad. Would some do so? Sure. How do I know? Because people do it already in spite of their religious leanings. The main difference would be that those who are oppressed are less likely to pray for deific intervention, and help themselves and those around them.