If you ask me, villains are one thing, complete monsters are another thing entirely. The former can be interesting and compelling, the latter are only good to cheer you up when they die.
Ivy killing people in order to protect the green is one thing: it makes her a good villain, like Two-Face. Ivy killing people (in excruciatingly slow and painful ways, which she cheerfully describes in detail) because it's her "guilty pleasure" is another thing entirely: it makes her an insufferable bastard, like Black Mask.