100% every time if a character kills my favorite they become my least favorite. I can admit he/she is a well written character but I still hate him/her.

With Kim, I already don't care for her. I'm not going to say she's a Sue. But she's ridiculously OP and always just so happens to have the right gadgets. While I don't hate her she really kind of annoys me with how very nearly perfect she is and how she can do pretty much everything.
All of this said, if she had killed Shego that would have done me in. She'd be on the list with my least favorites.

^I agree with some of that.
I mean, I do like Kim, but she really can do anything (if she can't, she'll master whatever it is by the end of the episode), and it becomes predictable and uninteresting. Shego is without a doubt my favorite character, and that rumor about So The Drama(maybe it's true?) sort of made me sick...
I'd definitely like her less if she killed Shego. I'd probably still watch the show, but I'd like it less as well.

I don't like Kim to begin with, but I probably wouldn't be interested in continuing watching the show, since Shego is my favorite and was my primary reason for watching (well, rewatching-- I of course watched this show some growing up).

Also she was almost definitely trying to kill Shego, despite the fact that she didn't succeed.

What MalloMar said.