US should not attack Iraq

I am curious about this - maybe someone will explain it to me.
What specifically is keeping Hussein from being assassinated - or is he already marked?
If not - what are the reasons he isn’t?
Is it an ethical issue? (dubious…)
Is it political? (policy? any type of government treaty?)
Is it practical? (too difficult?)
Is there a possibility of a bigger nutcase taking over?
I’m sure that there are clear reasons why this hasn’t been done yet… just wondering if someone could explain it to me.