I don't condone people abusing Yaz on social media whatsoever, but it's the attitude she's shown throughout - and before - this entire thing that people are taking umbrage with. Her dating Tristan - there's no issue there, as Liv as said. She wasn't even mates with Liv, so she's free to do what she wants with Tristan. Even with this "pact" thing Liv had with Tristan, still no issue IMO.
The problem with Yaz is that she's admitted to an emotional affair with Sam Prince while he and Inga were dating, long before the Corsica series. Then hearing all these things about her being tactile with him and touching his knee etc back in London - absolutely not okay, and the dirtiest thing a woman could do to a close female friend. And yes, Sam Prince is a low-life scumbag who no doubt reciprocated her flirting to the max; we're not forgetting that for a second.
Yaz and Sam are equally as hideous as each other. They brazenly ate each other's faces off in front of cameras, mere days after Sam's break up. They clearly got some sort of thrill from it, as they giggled about how "wrong" their actions were, before proceeding to eat each other's faces off again, then excitedly laughed about the prospect of being caught by someone.
Yes, many of the other cast members have done the dirty on their fellow castmates, but these two have truly excelled themselves in being the lowest of the low and showing zero remorse. The way they've been shamelessly flaunting their romance in front of the cameras and all over social media is incredibly distasteful, classless and shows a complete lack of respect towards Inga.