By worst, I mean most number of people killed and injured.

If not, I'd say in the modern era it's the Las Vegas shooting a few years ago. Maniac with an AR and a bump stock killed more than 50 people at an outdoor concert.
What did I win, Wanker?

Historians now believe as many as 300 were killed by the WHITE DEVIL.  More than 800 were treated for injuries.

but I think there’s a differentiation between “Mass shootings” and “riots”.

So just because there were a large number of shooters it's not a "mass shooting?"
PS: They've been exhuming some of the victims who were recently found at a mass burial site. Some were shot several times by more than one gun.

Yes. Criminologists differentiate between riots and mass shootings.

350 members of the Northwestern Band of the Shoshone Nation dead, making it the bloodiest — and most deadly — slaying of Native Americans by the U.S. military,
.
Danish immigrant Hans Jasperson in his 1911 autobiography, said he walked among the bodies, counting 493 Shoshone Indians dead

What do you guys remember about it? I very clearly remember Columbine. Virginia Tech. Sandy Hook. And that Texas shooting, Uvale or something where the cops didn’t do anything to help.  

The LV murder spree caused a huge amount of coverage for days. Wanker must have been drunk on home made dandelion wine.

Just because you’re brain dead that doesn’t mean anything.

Maybe you heard more than I did because I’m on the east coast. I seem to recall also being focused on a big project at the time that was keeping most of my attention. What details do you remember about it?

I haven’t looked back now for updates, but one of my memories of that shooting is that the investigation never turned up a publicly disclosed motive.  It’s led to some conspiracy theories about the shooting.

The shooter was a long-time gun collector who wanted to go out with a bang. He was able to check into a hotel next to the large field where the concert was happened and to load a huge arsenal into the room, including an AR with a bump stop. He started shooting a massive amount of ammo and killed nearly 60 people. Cops quickly figured out where the shots were coming from, burst into the hotel room and found him dead of a self-inflicted wound.
I don't recall the cops finding out anything regarding a motive. The guy was a stock trader of some sort who lived north of LV.
I did not do any research on this and the above is strictly from memory.

I went back and looked, and unless I’m screwing up my search, it wasn’t mentioned here much. Anyway, I just wanted to check. I’m about to post a larger subject about this.