We are not privy to all of the facts.
For all we know, and it's quite likely, that Soleimani was not legitimate per se, but was a Deep State operative or working closely with the Deep State.
It is perfectly just to put traitors and conspirators to death -- for the sake of the American AS WELL AS the Iranian people. If the good of the American AND Iranian people are both at stake, then YES, most certainly such an assassination would be justified.
The fact that no war has broken out between Iran and the United States shows that OBVIOUSLY there is something the average, superficial observer is "just not getting". Apparently, the superficial appearances are NOT telling the whole story.
Iran lobbed a few old missiles and purposely missed our soldiers. Trump hasn't sent so much as a single bottle rocket into Iran since. The conflict cooled off as fast as it heated up. Everyone's expectations were COMPLETELY thrown off. That's not the war-mongering that "certain Trads" have been calling for.
+1 for the Trump fans
...and yet another miss for the anti-Trump crowd
But some of us Trump fans are sick of being right all the time. So much winning, it becomes unbearable after a while :)