Ironically, it was the 'romance' part of the series that killed it.

As for the toxicity of it, Bella really comes off as extremely selfish and obsessed in some ways.
Like, as soon as she finds out/discovers that Edward is (possibly) a vampire, she suddenly wants to be super close with him.
She constantly asks to be turned, or asks what it's like being a vampire, or what she can do to be part of their world.
She wants to become immortal when she's only 17.
She thinks she's old when she turns 18.
It's a very...sad outlook on how Bella perceives things...
Or maybe it's more a sad outlook on how Meyer sees things...
Then when Edward leaves her for her own safety, she becomes incredibly reckless and an adrenaline junkie - completely disregarding the intentions that Edward had for leaving her.
I'm not saying Edward was any better, since he literally stalked her and trespassed on her property, and even watched her while she slept which is...verging on somnophilia...almost...which is another whole bag of worms regarding consent and such...
..I digress...