A 51% attack is against a specific pseudonymous target.
There are many kinds of 51% attacks, but I gather that is the kind you want to talk about.
If that happened, it would not kill Bitcoin, but would make people realize how powerful mining pools are and that they should be avoided.
As an aside, I think the most likely kind of Bitcoin 51% attack is one that rejects transactions with low fees. Since that would benefit miners in the long run, they might see no need to avoid mining pools. I'll be mildly surprised if this doesn't become a real concern in 2 or 6 years, when the block-reward halves.
In NXT, there is little cost if everyone wanted to attack a specific person, it would be quite easy. Mob rules. There would be no mining pools to blame. Nobody would know who is innocent and who is guilty because it only takes 51%.
As far as I can tell, what you have in mind is like 50% of Americans deciding to boycott Walmart. It's not likely. I'm not sure where you get "only 51%" from. 51% is a lot.
This is why anonymity is crucial. Nobody can be targeted if they can't be traced. The same thing goes for attackers, they are also anonymous. If an attack happens, good luck, they are behind 7 proxies.
You seem to be saying anonymity is a two-edged sword.
If a target was found out, and a lynch mob of at least 51% was convinced to attack the target, how would anyone know who to blame? If it was a business that managed to aggregate 51%, how would you even know it was an attack and not a normal orphaning since there are no IPs to compare blocks like GHash.io.
In Nxt, the blocks are signed by the accounts that forged them, so we'd know which accounts to blame. There's not much anonymity in Nxt. If a single entity gains 51% of NXT, then its true Nxt has a huge problem. I think an attack like you describe would destroy all confidence in the currency, so the entity would lose the value of their coins. It'd be very expensive for them.
There is no electricity cost to bring a bunch of miners in, just get a bunch of coin and attack your enemy. Then sell the coin anonymously. Your enemy is defeated and it cost very little compared to the same attack in Bitcoin.
Trying to buy 51% of NXT will be very expensive and will push the price up. Dumping 51% of NXT will crash the price, so they won't be sold at the price they were bought for. The attacker will lose a lot of money. The price crash would be a problem, even without the attack.
I think you are underestimating how difficult it is to get 51% of NXT. It's not like in Bitcoin, where entities have got that much hashpower almost by accident. In Bitcoin, you can get it almost unilaterally, by buying from ASIC manufacturers who have no stake in the currency. To get 51% of NXT, you have to persuade people who own NXT to sell to you. Likewise, to sell afterwards you'd have to persuade someone else to buy. Nxt has been running for 8 or 9 months now, and has had the third highest market cap for much of that time, but no-one has done the attack you describe.