As you all probably know, the US has made an airstrike in Iraq, killing Qassem Suleimani and several others.
I am not going to be sorry that Qassem Suleimani is gone, but as Mano Singham says, this is really really bad. Iran is not going to take this lightly.
Also, there is the whole problem of assassinating people – if this becomes widespread, it would mean that the US leadership would become a legitimate target for e.g. Iran. This is not a good thing, and is why most countries have signed up to use the international criminal court, ICC, to prosecute people instead – of course, the US is not a state party to the ICC.
If you want to know more about Qassem Suleimani, the New Yorker had a good portrait of him in 2013 The Shadow Commander.