There are ways to be critical of the Israeli government without being antisemitic. You could be critical of, say, something relating to unemployment insurance.
But that's not what people are usually talking about when this argument comes up - instead, they're criticizing Israel's right to exist or Israel's right to defend itself (which amounts to the same thing). When they're not being so explicit, they want Israel to embark on a policy which would lead to the end of Israel (accepting indefensible borders, the 'right of return', etc.)
In short, they're making the 'anti-Zionism is not anti-Semitism' argument. Personally, I think anti-Zionism is just the first type of antisemitism to try and simultaneously deny it's antisemitic. Let me try to explain with an analogy.
Let's say I was anti-France - that I believed French national identity was somehow illegitimate, and that therefore France shouldn't be a country. Since France shouldn't exist, France also isn't entitled to defend itself from external attack. However, I have no problem with French people! Great wine, great cheese - they just shouldn't be able to have a country. If a French person objects, I'll just patiently remind him that I'm not anti-French, I'm merely anti-France. Perhaps I'll criticize him a bit for inappropriately throwing out the anti-French card.
Sound ridiculous? Well, that's exactly how it sounds to me when someone argues that my people can't establish a state (in the same imperfect way most states are established) and then can't defend our state from attack. Everyone else's national self-determination is fine, but ours is somehow not.
So yes, when people make this sort of argument against Israel - a type of argument they would never make against Sweden or France or Mexico - in that case I do believe they're being more than a little biased against my people in particular, and therefore antisemitic.