If Bell's actually dead, that did come out of nowhere, because as far as I can recall, no major character has ACTUALLY been killed off so far. The closest we've seen is the one god they beat, who's still kinda alive within Yuki anyway.
I was actually really surprised to learn that the author is the same as Akame ga Kill! because I haven't ONCE felt like a character was actually going to die. Every time someone should... they don't. I think the author still likes the violent scenes, but for whatever reason (PLOT) they're just not killing anyone off in this series. Y'all who keep bringing up Akame ga Kill! we were GIVEN the tone in that series really early on. We KNEW what to expect. We're 106 chapters into this series, spontaneously murking a character like that is completely out of line with the tone established so far.
So yeah I doubt she's dead, the card probably "turns her into an item" and thus preserves her or whatever, so they'll be able to use some crazy OP healing ability to save her.