Most of what you said is centralized on the position that you cannot harm human existence, especially by atheism, and that history tells us that humans do all sorts of wrong doing for many different reasons. Some of the supposed 'bad trends' at the time (e.g., Elvis) turned out to be innocuous, while other seen-to-be-good trends at the time were later viewed as evil (e.g., Vietnam, Crusades). In the end, all is vanity under the sun (to use an Ecclesiates reference).
Well, the subject matter is very wide-ended to engage in any conclusive debate. There are millions of variables as well as incidences we could name, and each could be due to other (perhaps multiple) interpretations of those events. You will interpret my opinions as another Christian spouting off, and I will interpret your opinions as another atheist having no idea the consequences that secularism will have on society. Even if I were completely right and society feels more and more of the consequences of this 'unbelieving' attitude that atheism especially fosters, most likely there is no way that I could show a linear relationship between unbelieving-ness and negative consequences. We clearly cannot come to a consensus of opinion, not now, and not even if a hundred thousand more cited 'secularism inspired' incidences occur.
All I can ask is that you look with an open mind the huge number of terrible incidences that occur in society. Ask yourself if that behavior was ever recorded in the past. Try to determine what human need was trying to be met with each incident. For example, was the negative incident trying to meet a need for revenge, a need for recognition, a need for success, etc. What those needs tell you is that some individuals lacked meaning in their lives, and they tried to fill that need for meaning by going to extreme measures to obtain whatever ounce of meaning they could eke out of their situation.
Religious fundamentalism, nationalism, gangs, drug addiction, alcohol addiction, sex addiction, etc, are all attempts to meet this need for meaning. Healthy avenues such as sports, education, hobbies, etc are very limited in meeting this need for meaning. If the cup is not properly filled using those activities, people are apt to move to more extreme measurements to meet that need. The more they lack meaning, the more intense the desire to meet those needs. This, I believe, leads to bizarre and sometimes gruesome choices. The need to shock (sensationalism) is perhaps one of the most effective means to temporarily meet the human need for meaning.
Now, contrast that to a society that meaning is supplied on a dish that concentrates on spiritual progress and spiritual well-being. Such a society isn't caught up in bizarre and gruesome behavior. It is focused on a well-rounded individual. An unbelieving/secular society that is not controlled by the State cannot offer a competing solution to the spiritual approach. Rather, in such a society, it is every unbeliever for themselves. Humanism is perhaps one popular alternative, but much of society will not follow. Rather, they will entrench themselves in whatever satisfies the self (e.g., pornography, violence, etc). In a State controlled unbelieving society, they can offer such meaningful notions such as what Marxism has tried to do, but history shows that such programs do not work.
In any case, I doubt you or any other secularist will see my point. Fundamentalists will only distract from this issue by pushing for a return to fundamentalism, whereas this is only to abuse these human needs in other ways.
Warm regards, Harv