I wonder how seeious this think with Iran,and the US will become ?
Very.
Trump has effectively declared war on Iran. You're not supposed to kill the other side unless they pose an immediate threat. And that Iranian guy wasn't. Imagine if the French assassinated the British Chief of Staff because he was a bad guy. The British would be rightly pissed and get revenge.
Strictly speaking, American presidents are supposed to notify Congress before they do something war-like before war starts. Trump didn't. And the explanation he did send eventually to Congress was classified so, strictly speaking, the public won't know why he ordered it.
And since about the 1950's (the CIA were running amok in Sth America at the time), Americans (and American Presidents) are not allowed to assassinate foreign powers. Trump's done exactly that.
Iran won't respond directly. They don't need to since they have proxy armies in at least Syria, Iraq and the Lebanon. Already, there's been attacks on Americans in Iraq and Kenya. The Iraq parliament has voted to kick the Americans out. All the world powers except Russia, Turkey & Israel have diplomatically said it was a stupid idea. The oil price has shot up.
And it's only being, what, 2 days?