First time I read that, I interpreted it as "it would not be enough to prevent Harry from tearing the world apart". But there is another possible interpretation : if we consider that Quirrelmort was trying to turn Harry dark, and killing Hermione was part of that plan, then we can interpret it completely to the opposite : the killing Hermione won't be enough to turn Harry dark.

I don't give a high probability to the second interpretation, but it does stand on its own feet.