I just watched the first episode of It's Okay online with friends, and to be honest, the first episode was seriously not good for my anxiety, with all the blood and screaming and violence and the horror elements. So I'm wondering, do all subsequent episodes have a similar vibe, or is it a case of the first episode being more intense in order to hook you?

A lot of the episodes will have a bit of violence, dark elements like blood, creepy scenes etc. Basically yes, most of the following episodes will have that vibe. 

I stopped after episode 1 because of the butterfly wings. It was creepy enough when done in animation, but I was told that in episode 2 they showed her doing it to real-life butterflies. That's when I said "no matter how good it is otherwise, I will not watch this, and I will not support a drama that tortures animals, even if it's insects.

this drama is actually good if you watch it till the end

everything she does, happens for a reason and the ml is there for a solid reason.

everyone in the story has a solid story behind and is written greatly.

If these things are triggers for you, I'd give it a pass.  I was in the minority in that I didn't enjoy it.  I didn't like the FL lead, ever.  She never grew on me.