Yes, it seems to me if an attacker, who wishes to destroy bitcoin, gains greater than 50% control then bitcoins's in trouble, yes?

Let's assume the attacker has more than 50% but less that 66.666...% control - so can mine nearly twice as many blocks as the rest of the miners put together. What are the consequences? What can the attacker do? Is there a defense? The attacker can create bad blocks (double spends or whatever) but the other miners will reject the bad blocks. However,  the attacker keeps extending its fork of the blockchain, containing the bad blocks, at near twice the rate of the honest blockchain. But AISI, the attacker's fork can be shown to contain bad blocks. Can that fact be used, by honest miners,  to deter ordinary (non-miner) users from using the corrupt fork?