I don't think there are many people in the world that will mourn the death of Khamenei. He was a dictator who crushed or imprisoned anyone who showed dissent. He was brutal.
However, US policy in the US has been horrible in the middle east for...well...forever. Is it going to be different this time around? Doubtful. They have an idiot in charge now who entered this war with no exit strategy or plan at all. He is obviously making it up as he goes. They started the bombing campaign just when they were in the midst of negotiations with Iran. Now that Khamenei is dead, who will fill that power vacuum? And, if they don't get regime change, what will the outcome be? Keep in mind that the US basically helped create the grounds for Iran's revolution by installing and supporting the Shah.
Now, they've assassinated a leader of the Shi'a faith, which has large populations in Iraq, Azerbaijan and Bahrain. Will they be cool with that? I think not. Iran, as we know, sponsors terrorism. We know they will attack the US in some fashion. Maybe they'll have friends in those other countries help. Maybe the attack will be Europe or Asia.
The bottom line: war is never a good solution. We all know Trump is doing this to pretend he's a strong leader, but all it really shows is he is desperate and a terrible negotiator...