What's wrong with "religion"?

I think "religion" is too broad or nebulous of a term, to be useful for these discussions.

Even once we try to narrow it down, I don't personally have a strong objection to all religious cultures.

I think there is great positive utility and beauty in (for example) certain Native American spiritual cultures.

I also recognized that popular "Abrahamic" religious categories have a vast range of different beliefs, attitudes, and behaviors.
So I hesitate to lump those all together; despite how much they lump themselves together.

Most of the contention we run into about it
is provoked by
and aimed at
Christian fundamentalism and Islamic fundamentalism.
But even then, many people I would recognize as "fundamentalists" would deny that label; on conveniently technical grounds.
So then having a really meaningful discussion about it becomes very difficult, with people talking past each other because of using words differently.

As for which groups are spreading via violence (today),
I'd say most groups that self-label as "Christian"
and
most groups that self-label as "Muslim"
are psychological violent; without even realizing it.

I also think they indirectly generate a lot of violence;
especially if we recognize that the term "violence" could (and I think should) include all forms of serious injury.
Violence doesn't always involve a sudden burst of kinetic energy.
It doesn't always cause sudden or obvious physical trauma.

As for how that makes participants and supporters of those groups morally "lesser" than all atheists, or all deists, or all pantheists, etc?
It doesn't.
There is no shortage of non-theists with Major Personality Disorders.
Monster are monsters.
Trouble people are troubled people.
It doesn't matter to me what labels they do (or don't) hide behind.

But the reason there is a statistically higher percentage of covert predators in loosely-organized religious groups is:

The lure of special access.

Commonly, churches, temples, mosques, etc. maintain a platform of "authority" and of "spiritual leadership".
Stepping onto such a pedestal gives immediate un-earned level of trust
and access to extra-vulnerable young adults, children, women, the learning-impaired, the emotionally-vulnerable, the under-educated, etc..

The purpose of those positions is at least partly noble.

The expectation is that only those "called by God" (and thus vouched-for by God) will ascend in those positions.

But the reality is that it's super easy to "talk the talk", go through the motions, and convince the average religious community that you're "spiritual" and "a man (or woman) of God", and "called", and "ordained" for those "good works".

So then churches end up with the same problem corporations and political offices all have.
The people most likely to climb high into positions of power over vulnerable-others ... are covert predators.

Merely "being religious" does NOT make a person more likely to be covertly dangerous.
However, the "Divine Protection" religious communities magic-think is a real thing ... causes their masses to over-trust anyone wearing their group's sanctioned titles of leadership.

That, in turn, makes it extra-dangerous.

It accidentally but unavoidably turns religious communities into hunting grounds for predators ... while suppressing how free most members will feel to report abuses (lest they bring scandal to God's house), and thus limits those communities' ability to remedy those problems.

As for religions causing or worsening wars:
It's rare for any war to be directly waged for a religious reason.
And yet, literally all wars are waged for reasons that are virally propagated via popular religious groups.

It's like a basket of eggs.
Every egg represents a concept.
Religious groups pass around those baskets.
Some of those eggs are healthy and wholesome.
But some are not.
And religious communities are notorious for not knowing the difference. 

This is, of course, partly because most HUMANS are notorious for not knowing the difference.

So I'm not entirely blaming their religion.

But their church's claim to be the mouth and hands of a perfect Super-Being 
who assures guiding them into perfectly knowing the difference.
And yet,
their batting-average remains abysmal. 

Plenty of ideological grenades keep getting past around. 
And many of those come directly from their holy books.

People then pick up whichever concepts appeal to them.
Examples:
Authoritarianism (might makes right),
identity politics,
US-vs-Them,
the idea of disposable people,
the idea that anyone in the way of creating (or restoring) a more ideal kingdom on earth is "the enemy" and thus counts as one of the "disposable people",
"the ends justify they means",
a disregard for the ethic of consent,
etc..

Even the most popular versions of Christianity are covertly a form of racism.

They create an artificial social construct
which is then used as a premise for deciding who deserves to live longer and better ... vs who doesn't.

That's literally the core of their ideology.

With that, they delegitimize and defame all "others" as "the world" and "wicked".

There's even a case in Bible stories where Jesus felt that an outsider woman was thus too unimportant to help.
She had to debase herself to extremes, repeatedly, while begging him to heal her; before he finally agreed to help.

As he said, "I came only for the lost sheep of Israel".
It wasn't until generations later that his ministry was re-interpreted to be for everyone. And even then, the "offer" was still join
or else die (as disposable people).

The Bible even directly says that non-believers are worthless fools
who have "no good within them".
Thus, it threat-promises to eventually violently extinguish all non-members.  It promises the same fate for people in all the wrong religions. 

-Although, Big Daddy is supposed to take care of it.

The more destructive 'ways of thinking' in those religious baskets
are kept alive in the collective consciousness.
That, in turn, fuels wars.