the real issue with 51% isn't that big as many think, because no one in his right mind would spend a ton in mining equipment just to do a 51'ed to the network, it would be better for him to secure it in this case....
also if you take into account that the bitoin price is ridiculously low ,it is even stupid to do such a thing
This is valid if you think about a typical private entity. But a big private entity which can vew Bitcoin as a threat to their businness can do this. Also, govenrments can execute a 51% attack if they want (as well seize legit mining operations in order to facilitate this) without worryng much about financial losses.
the point is also, that a 51% can't kill bitcoin actually, because it would only mean that its price will restart from one/zero, like a new birth, thus making pointless do a 51 only for killing it, normally one would do it for the money...