If short (more precisely: containing less accumulated PoW) chain is to be killed, then why iXcoin, I0Coin, NMC etc. chains weren't killed by BTC chain? Why ETC chain wasn't killed by ETH chain?

On topic. I think such attack won't happen anyway. After ETH/ETC split ETH had overwhelmingly more hashpower than ETC, they even created a special pool for a 51% attack, still they weren't successful. It seems to me people generally prefer not to engage in such malicious activities. Additionally, conducting such an attack could trigger PoW change in the attacked chain. That would render expensive mining equipment useless should market prefer the minority hashpower chain. If they abstain from attacking, they keep possibility to switch to the chain preferred by users.