should I stop being her friend?

Basically me and two other girls were best friends, when one of them, who I’ll call L started being sooo b!tchy. She constantly slags off people, says she hates them, and is generally just horrible to people who’ve done nothing whatsoever to her, but then she’ll talk to them later as if she loves them.

She did this for ages and me and the other girl (I’ll call her S) decided we’d had enough and if she didn’t stop being so horrible to other people, we’d stop being friends.

So we did over the summer, when one day L decides to say sorry to S and completely ignores me. I wasn’t too bothered, but just mainly confused as to why she thinks I dont need apologising to. Anyway, we went back to school and she realised she’d be better off speaking to me and we ended up friends (ish) again.

It went alright till about now, and shes starting to be horrible again. Shes so horrible to people, and even me, behind my back. S is my best friend ever and I know she’d tell me if she said anything, and L’s been slagging me off behind my back to S. She does it to nearly EVERYONE and it’s getting on my nerves so much. She just CAN’T be this horrible to people and expect them to talk to her when there isn’t anyone else.

Like once, a girl was crying because her aunt has cancer, when L just blurted out ‘OH JUST GET OVER IT, I’VE HAD TO. NO-ONE CARES’ (someone in her family had cancer). It was SO uncalled for. Sure, she might be upset? But my grandma died of cancer and I’d NEVER say that to ANYONE in that position, no matter who they are.

What should I do? Why do you think she’s so horrible to everyone? (sorry if this is long!)