If her gums and tongue are almost white, she is anemic and ill. That's why she's so ill-tempered, she doesn't feel well! Take her to the vet and get her treated, there is no reason to put a FeLV cat down as most of their issues can be treated, same as in a cat that is not FeLV positive. You really need to give her a chance, and get her to the vet. Just putting her to sleep isn't really a very good option, if she's got an issue that can be treated. Cats can live healthy and happy lives with FeLV, it's not something they need to automatically be put to sleep for. And no, taking her to a shelter isn't an option as she'll be euthanized as soon as you turn and walk out the door, since FeLV is highly contagious especially in a shelter environment.