I just read every strip Malaya's been in and I still don't get why they hate each other.
Because Malaya's very existence calls upon Sal to share Marcie, the only
possession that is uniquely hers that she's ever had. She isn't happy with that fact. I use that word deliberately, by the way. In my view, Sal's issues tend towards possessive behaviour and she doesn't really always think of Marcie as a person with her own desires, just her one relationship anchor in the world.
I think that this is the turning point in Sal and Marcie's relationship. All Sal had to do was be
neutrally civil to Malaya but she wasn't even able to do that. No matter how grating she finds Malaya's personality, it's clear to me that Sal's real issue is that she doesn't like sharing Marcie with
anyone (she similarly abrasive with Carla who is more mature than Malaya and shrugs it off). I think that Marcie was watching this confrontation long enough to realise this. I'm expecting her to tell Sal that her possessiveness is poisonous and that she's cutting her loose.
Sal has a string of issues but Malaya hit the nail on the head during this confrontation. In the end, Sal wants to be liked and admired but
only on her own terms. It's all take for her and very rarely does she unbend enough to contribute positively to a relationship. She can and does do so occasionally; she even can be generous and kind to people in need. However, in general, she's desperate to put across this air of independence and toughness that she seems to have based on a 1970s TV action show and it just turns people off of her.
Sal has issues that makes friendship, especially the reciprocal part of it, enormously difficult for her. Yet she manages it on occasion; she just needs to be motivated to try harder. Maybe Marcie not wanting to interact with her anymore will force her to broaden her horizons and try to address her own problems too.