It's definitely worth delving deeper into this moment. Certainly the Alternian view of the situation would agree with you - most trolls would condemn Tavros for failing to kill his tormentor in the grand tradition of the Empire.
Even from a human perspective, you could easily make an argument that Tavros did something wrong here. Vriska was literally asking for it, and she experienced a painful death that he could have prevented, had he took action.
However, I don't personally agree, for two main reasons.
Firstly, Vriska was bleeding out on that bed because she chose to be. She actively prevented Tavros from kiss-reviving her, while perfectly aware of the consequences for doing so. Tavros already tried to show mercy to Vriska, and she stopped him.
In essence, Vriska refused an antibiotic from Tavros, and then begged him to amputate her infected leg with a rusty saw.
Furthermore, every single time Vriska has tried to turn Tavros into a 'better', more violent troll, he's refused to comply.
She tried to extract a mercy kill from him - again, in a situation she engineered, and could easily have prevented - while fully aware that he isn't capable of performing mercy kills, and probably never would be. What, exactly, did she expect?
I think this is a good take, and might the real reason she pulled Tavros into this. I've pointed out before that Vriska clearly knew about her Quest Cocoon before Aradia's attack, so she could have gone there before, and ascended in much less dire circumstances.
She didn't. She is just a kid, and was likely terrified at the prospect of taking her own life - so she tried to put her life in someone else's hands instead. But, because she's Vriska, she picked the worst person she could possibly have chosen.