The downside to atheism is that there are no set morals of any kind; since there's no God to judge us, everyone sets his own standards, and so having a society devoid of religion wouldn't be society at all; if you think killing someone is okay, then it's okay. If you think stealing is okay, it's okay. And of course, who put the country's leaders in authority? Or the police? Or your school teachers? What right would they possibly have to boss you around when the only rule is your rule? Pure atheism would result in anarchy and chaos.
Okay, let me tell you this is utter bullshit.
I'm an atheist. Pure and simple. I do not believe in any god and I negate the existence of any divinity. Yet
somehow I still have moral standards! I believe rape is wrong, murder is a crime and paedophilia makes me sick to my stomach! HOW ABOUT FUCKING THAT I'M A DECENT HUMAN BEING BECAUSE I ACT LOGICALLY AND CARE ABOUT PEOPLE.
The Bible says a lot of dumb things. That rape = marriage thing, the huge promotion of incest, the complete condemnation of homosexuality, the 'no shellfish' rule no one follows, the cloth rule, the crop rule. Hell, it even says bats are birds. You can't except me to base my entire belief system on something so obviously morally and scientifically flawed.
No true Christian believes they are perfect; the Bible clearly states that ALL have sinned and fallen short of the glory of God. Also, the Bible states that being such a zealot (such as killing people just because they are sinners) is wrong, so they aren't actually doing it in the name of God.
As for the authority part, God's rule IS to follow our leaders. The Bible says that such people DO have authority over us and we are to follow them as long as they don't contradict God's pre-set laws (for example, if the President declared a law that said "go kill your neighbor," we wouldn't follow it). God is not an anarchist; in fact, He created society.
In most decent countries, Presidents have legalized homosexual marriage. This is the case of the country where I live (where it wasn't a president, but the king and the PM).
Time to go rip them a new one for contradicting God's Law.
If God does not exist, where did these widely accepted ideals such as murder being wrong come from, especially when the result of a certain frowned-upon action would not truly have an effect on the population at large?
Because if the only reason as to why you're not comitting murder is fear of reprisal from a god then you have serious issues. Normal people don't kill other people because they realize it ends another person's life and makes many others suffer, amongst other things.
Decent people don't like it when others suffer and it has nothing to do with deities.
What drove man to determine these specific morals, if man even made them?
Some of them were based on the Bible, but since the Bible wasn't written by god but by the apostles and some other guys it's perfectly logical to assume they infused it with their own opinions and passed it off as their god's. Some of them are logical: shellfish used to be very unhygienic and caused a lot of death by food poisoning because people were unable to wash it properly or keep it cool, anal sex was also unhygienic due to improper cleaning methods (condemnation of homosexuality), etc.
And some laws have changed as times progressed because people realized they don't need to blindly folllow a moral rulebook written 2000 years before their time.
Why is it that someone who thinks mass murder is right is clearly messed in the head?
Because they are most likely sociopathic or have some other kind of disease that creates apathy towards human life. This is a clinical condition and has nothing to do with god or a holy book.
(and no you fucks, don't start self-diagnosing with this because you're '2 kool to care about others' or I'll kill you)
And most importantly: why do people feel guilt if moral is determined by man? If it wasn't determined by man, how could evolution give us guilt?
We feem guilt because we're capable of intelligent thought, and as such of empathy (unless one has a disorder, as previously mentioned).
I'd answer better but I'm late for school as it is.