It's one thing to sacrifice yourself; it's <em>your</em> call. No one else is hurt but you. It's a totally different thing to sacrifice other people, to hurt others in the name of your own "cause." Here, Katniss recognizes it as "coldhearted." Despite her strong sense of justice – or, perhaps, because of it – it's the kind of activity she doesn't want to be a part of. It's bad enough to have one person sacrificing himself for her. She doesn't want others to have to do so too.