This is a case of "hate the game, not the player". Out of context, what you're saying is correct, nobody should say "israel has a right to exist". In context, lay off him, this is wrecker behavior. You're shitting on the only Muslim candidate in this election (and the only one with a history of supporting Palestine) for not falling into the obvious trap that an aggressive and hostile media class tried to berate him into. Answering "no" would have been just falling for the trap and would have been a stupid thing to do. They already hate him because he supports Palestine. It would have been cool if he said "death 2 isnt'real, death 2 ameriKKKa, unlimited genocide upon the first world" but it would also have been stupid to torch his entire political campaign just to take the bait.
I didn't like his answer either but he gave the answer a politician in his position has to give and he still refused to play along. They'll call him antisemitic anyway, but he didn't give them a soundbite to play over and over again to smear him with. Have you dealt with manipulative snakes before? When they try to get you to hang yourself, you don't help them.
Bernard "separate but equal" Sanders