I completely agree with you. I think, and I have said several times, that Voldemort is JKR's one artistic failure, a supervillain from central casting - snake eyes? check; long skinny fingers? check - and not even very impressive of his kind. Marvel Comics have twenty regular supervillains that are more interesting. Umbridge, on the other hand, is an absolute triumph, a person whom we would recognize and hate anywhere; she must represent issues in personality and social role that JKR feels very intensely. The fact alone that she is a Ministry inspector, and that JKR has been a schoolteacher: just ask any British schoolteacher about their views of the Ministry, then stand back and wait for the hurricane.

As for the frightful humiliation she suffered at the hands of the Centaurs, yes, that may scare her off for a while. But does she strike you as the kind of person who learns from adversity, or who is even scared off her own vengeance by danger? In my book, it means that the Centaurs are now marked down for vengeance. And their own vicious arrogance makes it likelier that they will get some severe sort of punishment from the humans they murderously despise.