The problem with being anti-Israel is that it implies pro-terrorism. I agree that the situation the Palestinians find themselves in is intolerable, but you have to accompany any criticism of Israel with some appearance of fairness, e.g. condemning the practice of sending teenagers into Israel with homicide vests and killing innocent civilians. If you're monolithically anti-Israel without any trace of criticism of the terrorists, this is when you risk being compared to the Nazis by righteously angry partisans.