the one killing citizens who speak out against it

Truly, the most dastardly invention of the Iranian government was killing people who oppose it. No government before or since, especially not in the West, has steeped to such lows.

In various circumstances critical support of problematic governments is support of the people when harm to their state by outside actors will bring harm to those people. Most communists have a general understanding that the way Iran is today is in fact America's fault and that the change it needs won't come from outside of it, particularly when the people using various problems as a political weapon do not have the improvement of the lives of the people as their goal but instead various other geopolitical and resource interests.

The most recent historical example of this would be Syria, with Libya a close second and Iraq a close third. All of which are objectively worse off thanks to western interventionism.

You can and should oppose interventionism and outside actors fucking with the situation there if you do care about the people, while also not defending the theocracy and support real local political movements for change (ie the ones not funded by NED or various other cia or nato affiliated intermediaries).

I think a way to do this without supporting oppressive regimes is to specifically support the people, and not the government.

On Hexbear we have seen this line of reasoning a hundred thousand times and so we just laugh now whenever we see it; I thought you were making a joke until I saw your instance.

The cause of so much of the suffering of "repressive regimes" like Iran, Cuba, Venezuela, Syria, the DPRK, etc is specifically because of the sanctions that the West puts on it that are designed to impoverish the people and try and make them overthrow their government, because they refuse to engage in the global economy according to the United States's rules, and not really because of those "regimes" themselves. Of course, it's taken for granted that what the United States wants is what everybody should want, but considering the billions being exploited abroad for tiny wages in hostile working environments for the West's benefit, perhaps America's "international rules-based order" isn't the best for anybody except for the West themselves! Of course, America has all the military bases, and those countries do not, and bullets and bombs tend to be quite persuasive.

For liberals, which I assume you are, these sanctions exist in a weird doublethink space. Working through it, liberals basically end up saying something contradictory like "The suffering that the people here are experiencing is because those countries are Bad. We need to put sanctions on Bad Countries. The sanctions aren't what's causing the suffering, it's the Bad Countries' fault (which thus implies sanctions don't work and have little to no effect), but we still need to put sanctions on them to punish them (thus implying that sanctions do have some negative, disciplinary function)."

Sanctions both do and do not function depending on the rhetorical frame you're taking at any particular time. When you're talking about the repression that Iranian women feel and why that sparked the protests, the sanctions will never be mentioned - this is purely Iran. When you're talking about the fact that Cubans struggle with food insecurity and don't have enough fuel and sometimes some of them protest or complain, then what caused those shortages is, again, never mentioned - it's purely the Cuban regime. If, on the other hand, you're talking about how repressive regimes must be punished in general, then westerners online clamour and shout for sanctions, sanctions, sanctions.

This is why we laugh about such "support the people, not the government" rhetoric a lot of the time. Of course, in the case of Iran and similar countries, they aren't left-wing and so we only really have critical support (in the sense of "they are better than those they are opposing, but they are not good in a vacuum") and there is genuinely nuance about how the Iranian bourgeoisie are worsening conditions by exploiting the people, and repressive religious institutions, etc, but by and large American sanctions are the larger factor. In the case of Cuba, or the DPRK, such a line about supporting the people, not the government is quite ridiculous. Liberals (usually of the chud variety) who just come right out and say what they really mean - that, yes, the sanctions are explicitly designed to make the population overthrow the government so that Western compradors and corporations can loot it of its resources and exploit its people - are horrific monsters, but at least slightly refreshing compared to the mental knots that most liberals tie themselves in to not say that line explicitly, invoking "restoring democracy" and "fighting authoritarianism" and other such meaningless cliches instead.

What makes this antisemitic? As far as I can tell, the issue is that the other competitor was an official representative of Israel, not that they were Jewish

Israel has a history of employing incredible violence against Iran in the most literal sense of that phrase.