Ehud Barak is an Israeli leftist. Like leftists everywhere, he will do or say anything to gain/retain power. In this case, he gains by 1) opposing Netanyahu, which is red meat for the Israeli left, thus 2) distinguishing himself to leftists abroad as the guy who is not the “bellicose warmonger” (ie: so he can get campaign cash from the EU, Stanley Greenberg to run his campaign, etc.)
IIRC, Barak is Netanyahu’s current defense minister and is a hawk on Iran totally aligned with Netanyahu. Barak burned many bridges to his labor party a long time ago.