A good (read kind, empathetic, and tolerant) person who is a Christian, Jew, Muslim, Buddhist, Hindu, Shintoist, Sikh, Baháʼí, Zoroastrian, or whatever and deconverts will still be a good person, just a good person who is now an atheist or agnostic; such people, when they leave religion, tend to embrace the philosophy of secular humanism, whether or not they identify with that term, as its basic tenets are quite similar to religious ethics at their best (ideas like Christianity’s Golden Rule, on which practically every major religion has some close variant). A bad (read cruel and/or bigoted) person who is religious may remain a bad person after deconverting, and is probably much more likely than a good person to embrace a malignant non-religious ideology such as Stalinism, Maoism, Objectivism, or a secular form of fascism (fascism is usually religious, and often explicitly so, as in the case of the Franco, Mussolini, and Pavelić regimes’ close alliances with the Roman Catholic Church, but it doesn’t necessarily have to be — worship of the nation can entirely replace worship of a deity or deities, and provide just as much excuse for bigotry and cruelty).
However, one can also find plenty of ex-believers who spoke and behaved in cruel and bigoted ways when they were religious because they were taught that cruelty and bigotry were their religious duties, only to renounce those actions and seek reconciliation with those they had harmed after deconverting. People who are kind by nature but are part of religious communities that make a virtue of hatred and wish suffering on all who are outside the community or violate its arbitrary moral strictures will tend to experience cognitive dissonance between their own desire for others not to suffer and their religious leaders’ assurances that God intends to inflict infinite suffering on everyone who doesn’t toe their particular religion’s line; that sort of dissonance is a common catalyst for deconversion, perhaps even more common than studying science and discovering how poorly it comports with religions’ claims about the natural world.
On the other hand, I think one would be very hard-pressed to find an example of a saintly or even just basically decent religious person who became a worse person after leaving religion for atheism or agnosticism, let alone one who demonstrably became a worse person because of leaving religion for atheism or agnosticism. I suppose its possible that the works of Ayn Rand have achieved a few such malign deconversions, but among Americans at least Rand’s real trick (not that she’d have wanted it this way) is that people tend to ignore her atheism (ironic, since her vitriolic contempt for religion makes the criticisms of Richard Dawkins, Christopher Hitchens, Sam Harris, Madalyn Murray O'Hair, etc. seem mild by comparison) while embracing her socioeconomic delusions. Many of her acolytes, such as Paul Ryan (R-Me-Me-Me-Me-Me!) and Mark Sanford (R-SC-by-way-of-the-Appalachian-Trail-or-maybe-Argentina), have whole-heartedly embraced the worship of Mammon and the “virtue of selfishness” while still thinking of themselves as Christians.
Many decent Christians might argue that they are mistaken in thinking so, but that looks to me like almost as much an instance of the No True Scotsman fallacy as bigoted fundamentalist Christians seeking to deny the Christianity of tolerant liberal Christians. Trying to impose any definition of Christianity that excludes some people who claim to be Christians is a mug’s game. I will say, though, that if I were a believer in the Bible, I might observe that the term “antichrist,” as it’s used in the very few bible verses where it occurs (none of which imply that it refers to only a single individual, and one of which explicitly states that “now many antichrists have come”), fits Ayn Rand as well as it fits any human who ever existed, precisely because her avowedly anti-Christian philosophy has proved so appealing to so many Christians.
My bottom line is this. If you are religious and a decent person, I’m not interested in persuading you to give up your religion. It probably doesn’t make you a better person than you would be without it, but if it makes you happy that’s fine by me as long as you’re not trying to proselytize at me. However, I would like you to consider how defending your religion from criticism may give cover to both those who use religion as an excuse to harm others (and would just find another excuse if they weren’t religious), and those for whom religion is the actual reason they harm others and who might become better people if they stopped believing. I would like you to consider that saying “but religion is good!” (even if you do remember to add the caveat “for me” or “for some people,” which some of you don’t always do) can itself be harmful to those who have been seriously scarred by religiously-motivated abuse.
I would like you to consider that anti-religious anger nearly always exists in response to religious abuse, and deserves far more latitude for that reason than religious attacks on the non-religious. There is no more symmetry between secular anger at religion and religious attacks on the secular than there is between Black anger at White people and White racism, between feminist anger at men and misogyny, or between some radical LGBTQ activists’ expressions of contempt for the cishet majority and homophobia and transphobia. Each axis of privilege/oppression runs in only one direction, and make no mistake, in America today the religious are still the privileged and oppressive side relative to the non-religious, even if the unrepresentative microcosm of a DailyKos comments section might make it feel otherwise.
And finally, if someone you know, a fellow decent religious person, is in the process of deconverting, I would like you to consider that no matter how important your faith is to you, no matter how sure you are that it makes you a better person, no matter how insecure it might make you feel that someone you trust and respect who has shared that faith in the past is considering discarding it, you should respect their decision. I would like you not to try to change their mind, to recognize that the loss of their faith will almost certainly not change who they are as a person, and to realize that even if it does that change is at least as likely to be for the better as for the worse.