Is anyone claiming that this isn't a reasonable stance for them to take?
Like, plenty of people I'm sure don't want them to succeed, but I don't think anyone is a shocked Pikachu that they want to.
Iran hasn't been historically shy about calling for people's deaths, and as you say, their head of state was just killed. Of course they want to retaliate. That is the natural and expected thing for them to want.
It may or may not be a good thing if they're able to succeed, depending on your perspective. But I don't think anyone thinks it would be an "unfair" thing of them to do.