Who is the ETH killer?

Could you please elaborate on the concept of an 'ETH killer' and discuss some of the potential candidates that are being seen as potential threats to Ethereum's dominance in the cryptocurrency space? What are the key factors that contribute to a project being labeled as an ETH killer, and how do these projects aim to differentiate themselves from Ethereum? Additionally, how do market trends and investor sentiment play into the rise and fall of these potential competitors?