I fail to see why this issue is about Israel's legitimacy. OK sure there are people about who do not recognize the state of Israel. But when some European state or a representative of an international body (that very clearly recognizes the state of Israel) says "could you pretty please kill a few less civilians?" how the hell does it turn into a question of Israel's legitimacy? It's like saying that criticism of the US war in Iraq is a question of US legitimacy. In my opinion it is just another way to try to bolster support: "stop asking that question, it means you are for the destruction of Israel, now be quiet".
NATO backs Turkey's fight against ISIS (Reuters)
from the article: IDF legal official: Israel should probe Goldstone Gaza report