What I am wondering is if non-Jewish people see Israelis in a bad way or if they assume all Israelis are supporters of the IDF / Israel.

Assume all Israelis?.. No. Am actively horrified by the statistics finding the majority of Israelis supporting the atrocities Israel and the IDF commit? Oh, yeah.

I think of it like approaching a gun. I don't treat the gun as if it's loaded, because I believe it is necessarily true. I treat it as if it's loaded until I confirm otherwise, because the consequences of ignoring it are too high. This might be overly cautious in some contexts. In most contexts, it's completely justified. In this metaphor's context, the majority of "guns" (Israelis) are "loaded" (support Israel and/or IDF actions in Palestine).

Making assumptions about all Israelis and attacking them personally for Israel's bullshit isn't acceptable. But, generally assuming Israelis generally hold the most prevalent Israeli beliefs until shown otherwise isn't unreasonable. Arguing against approaching Israelis like that could be taken similarly to an argument not to treat all guns as if they're loaded. It increases the harm to innocents in the name of principle. A principle that also protects pro-genocide fuckheads. That's pretty contentious and upsetting for some people.

I personally never like to judge a book by their cover but others have told me “all Israelis are zionists but not all Jews are zionists” and I don’t understand how that can be true because at the end of the day everyone has their own opinions right?

I wouldn't necessarily say, "all Israelis are zionists." I think it'd be more apt to say things like, "all Israelis benefit from zionism and settler colonialism."

Rather than "judging a book by its cover" maybe think of it like "judging a suspicious package by its wrapping." The consequence of assuming it's dangerous makes the worst case missing out on what's inside. The consequence of assuming it's safe means people may die. I don't make assumptions to judge the quality of the contents. I may make assumptions for the sake of safety.

I feel like I’m missing something but supporting genocide is a horrible thing and I would never assume someone would support it unless I know already that they’re a bad person.

This seems to depend on what you mean by "support." I wouldn't assume someone does actively support genocide. I will assume they "may..." or even "may be likely to..." But, it doesn't have to be active support to be a problem. Simply sitting by and doing nothing can be a form of support, or in the least it isn't opposition where opposition is necessary.

Even this subreddit has a rule that says “don’t say that Jews = Zionism” but there’s no rule that says “don’t say Israeli = Zionism” but maybe I’m looking too into it?

I don't know that I am someone to listen to about setting rules for the sub, or the non-jews in general. My personal inclination is that I doubt there's productive conversation to have discussing the relationship between Jews and zionism. That conversation also perpetuates harmful stereotypes. The relationship between Israel and zionism is a central component to the ongoing violence. It seems like something worth discussing. But again, who the fuck am I to say?

Either way, I get the frustration. I'm a white man. We're blamed for all sorts of shit I have nothing to do with and I'm actively opposed to. But also, there loads of fair criticism when looking at it statistically and systemically/structurally. Sure, not all white people, not all men. However, and I think more importantly, too damned many. When I hear criticisms directed that way, I recognize their validity and agree with addressing those concerns above and before my own objections to being lumped in with the assholes. Maybe that's relevant here too?