Zionism says 'we, as a people, would like to live here. We'll buy it if you're selling'. Anti-Zionism says 'we should drive all Israelis out of Israel.' One of these is harmless. The other is attempted genocide. Fortunately, Zionism has pretty much won, but some people instinctively root for an underdog, which anti-Zionism is nowadays (thankfully).
I oppose Zionism, but I would not support driving Israelis out of their homes. Nor would most anti-Zionists. Though Palestinian refugees should be allowed to return, and something should be resolved about how there are people living on their (stolen) land. I'm unsure about what should be done about the settlements.
And to chalk up opposition to Zionism to "instinctive rooting for an underdog" is stupid. You're basically doing the arrogant "hurr hurr everyone who disagrees with me is just a dumb little teenager" thing.
stop agreeing with me, you're making me reconsider my position.