Just to play devil's advocate, it's possible that the anger is coming from a place that actually isn't a problem. It sounds like the readers really care about the character. You've struck an emotional chord with them, but it's creating a powerful reaction. If it means they never read another book by you, that might be a problem, but if your intention is to strike that chord, you've succeeded.

Now there could be logical reasons why they're mad at you (the fridging problem is an example), but if it's purely an emotional response based on their love for that character, only you can decide whether potentially pissing them off is worth making them feel that love and loss. The trick is finding out which it is (and what to do about it).