I don't agree with the more cynical takes here, but what religion does is provide a set of guidelines that should be unassailable. Even for those who go astray of those guidelines, there is an agreed upon rulebook that everyone follows. Now you will sometimes have some minor interpretation differences, but the core philosophy is generally agreed upon.
Secular humanism's only boundary is what we have not convinced ourselves is good yet. Our move to everybody going their own way has been a disaster. It plays out in the news daily.
I disagree. Most of the core philosophy found in the major religions is the same-the Golden Rule, most of the Ten Commandments, focus on love, not hate, etc.
Most secular humanists believe in that core philosophy as well (I do). I think that must be partly human nature as a social species (there are all types of game theory explanations for morality, for example). The difference is that the secularist seeks a rational, naturalist explanation instead of a supernatural one.
As for everybody going their own way, read Pinker. He analyzes violence stats, etc. and says we are actually becoming
more peaceful. We are also, in the West, becoming more tolerant of differing lifestyles, etc. To the extent it's just about tolerance, I'm all for it (and I bet given your past posting history that you are, too).
It's when we cross over the line and think tolerance means equal results for all that is the problem, in my mind, mostly because it ignores reality--that is, the natural world. I don't think it's any coincidence that anti-racism, hard trans activism, etc. deny reality in favor of more religious-type ideology and thinking, some of which borders on the supernatural.
As for the mass shootings, the underlying cause here is that we have a lot of very, very disturbed and marginalized people with access to increasingly deadly weapons and technology. It's not either one or the other of those two things, it is both.
I think what religion might do to combat this is give these people a place to feel like they are not marginalized by society--that they are loved and valued by a community or a supernatural being (I think you need the former to feel the latter, but I could be wrong). But even that might not stop the truly mentally ill (like the Sandy Hook shooter, and maybe this Uvalde shooter).