I don't think a successful 51% attack would kill Bitcoin at this point nor do I think anyone capable would be willing to attempt it.
While - at least on paper - 51% is not impossible, if that would happen it would be a blow for Bitcoin's credibility and the way of handling it could keep it alive ... or not (i.e. indirectly killing it). But it's a way too hypothetical situation and imho OP's question/problem was caused by lack of knowledge (and hopefully that was handled too).