First, if the US does not unilaterally rip up the international agreement with Iran regarding nuclear weapons and impose sanctions not only on Iran but secondary sanctions on countries who do business with Iran then there is no reason to believe there would be this level of tension.If it comes out to be true that Qassem was planning more attacks on the US, then I don't see any other way for the US to proceed without taking him out. If it's a lie, then Trump is the moron we all think he is.
There's zero way to know right now.
Secondly, if the Administration has evidence the Iranians were plotting attacks on US forces then there is no reason to withhold such information from Congress or our allies prior to acting.
Thirdly, assassinating QS cannot be seen in any other way than a declaration of war by the US against Iran. It's not logical to claim this action is preventative when the clear effect of this action will be retaliation.
Lastly, Trump is a fool and he is going to get Americans killed due to his foolishness (cue his bravado about war with Iran that conjures memory of Chaney and Rumsfeld regarding Iraq).