Mostly, I'm surprised that Iran went into Iraq and stayed there, and started having control over the Iraqi government and militias; and receiving $$ from Iraq for doing it (getting around sanctions). These two countries fought two awful, bloody wars with each other.
And that's a result of us going into Iraq in the first place -- and mostly but not entirely leaving.
Or (the optimistic view) is that we don't have the full story. The timing and overt reasoning of this seems like we're taking advantage of the Embassy provocation -- finally saying enough is enough with this guy.
And at the same time there's massive protests against Hezbollah in Lebanon, and Iranian involvement in Iraqi governance, and in Iran (their internal protests). This assassination could be also seen as taking advantage of those developments. I mean, even in the short term, there's got to be some destabilization of Iranian operations in the region and the message that Iran isn't the local power they want to be. I don't think the President (or any President for that matter) comes up with the pluses and minuses of acts like these in a vacuum. He's got to be advised by somebody.