Why now, why this?

It is taken as read that Qasem Suleimani was engaged in actions that led to the death of many Americans. How he might be held to account is a complicated question (unless you are one of the mind-numbingly infantile Fox News crowd from whom everything is "real simple"). He was a government official in another country; it is unlikely that he would ever be held accountable in the US, although a case might be brought the ICC. Trump, who does no acknowledge that any supranational body has jurisdiction will argue that since he can't be brought to justice in the US, extrajudicial killing was the only option. But holding Suleimani to account for his past is not what the administration is arguing was the justification for the attack. It is claiming that it was necessary to prevent an imminent attack on US persons, and thus was permissible under the Authorization for Use of Military Force (AUMF) or the War Powers Act.  That's the legal justification for not consulting Congress. A Congressional declaration of war would have been not only unlikely to pass but would have made any covert operation impossible. So an AUMF- or War Powers Act-based authorization may have been appropriate.       

If so, two question need answers. First is why now? So far the administration has been circumspect as to the nature and timing of the imminent attack. So we need to know what the intelligence community concluded was being planned and when. This gets messy, first because the question only arises because the AUMF was invoked. And its use is contingent on the choice of action to be taken to respond to Iran's escalating meddling

The second question, and arguably the larger one, is not whether to have struck back, but how. There is a good case that to do nothing in response to the killing of an American contractor given the history of Iran's and its proxies actions (the strike against the Saudi refinery, the mining of tankers in the Straits of Hormuz, and the downing of a US drone), would be taken as weakness and a license for Iran to push further in an effort to extract concessions on economically crippling sanctions. Diplomatic channels, for whatever reason, were no longer a viable means of curtailing Iran's activities. That is largely a function of a choice made by this administration in favoring confrontation and threats of force over diplomacy.  But the question of whether killing someone who was the Iranian equivalent of the Director of National Security of the CIA Director was the most effective way of dealing with Iran's posture is critical. Likely a menu of retaliatory actions were presented to Trump - indeed it appears it was something Pompeo had been discussion privately for some months - and it is possible that this was presented to Trump as a target of opportunity, an opportunity he took thinking it would help him win the next election. It is almost certain, given what we know about his management "style", that he made the decision impulsively with no clear understanding of its ramifications. As Oliver Hardy might have said "Well here's another fine mess you've gotten us into".