Congratulations on realizing that we have a bit of a colonial past to reckon with, I guess?
Anyway, I think you'll find a broad overlap between the group focusing on anti-Zionism and those that
already do point the finger at this country for its record of colonialism. However, as to the "shouldn't exist" argument, I will say a couple things, since people who aren't steeped in this discourse tend to approach it in the clumsy manner of one who has heard something scandalous as they walk by a conversation, misinterpret it since they don't understand the context in which it was said, and then get outraged anyway. First is that there is the obvious difference of over 150 years and all of the systemic global changes that occurred between the founding of the USA and that of Israel (which, should be noted, was established as the rest of the world was already decolonizing elsewhere, and still within living memory). Second (and probably because of the time difference and the relation of America's colonizing with so many other parts of the world at similar times), the definitions and rhetoric used tend to differ between the scenarios. When it pertains to the USA and the Global North in general, the
systems are usually referred to, like capitalism, racism, etc. rather than the countries involved. In my experience, the discourse around Israel/zionism/colonialism, however, tends to refer to
Israel systematically in and of itself- and in the circles Israel is defined as a Zionist/colonial project which inherently favors some peoples over others, or as noted radical... *checks notes* ...uhh...John Kerry...said: "if the choice is one state, Israel can either be Jewish or Democratic, it cannot be both" (I understand you will disagree, but understanding this requires understanding the definitions they use). The statement, then, that "Israel shouldn't exist" is in that manner more like saying "end capitalism" in that the focus is on the systems rather than on the individual actors.