It IS an act of asymetric warfare. That part, I don't think that most folks disagree with. So are the IED's that take out American soldiers. And a lot of other devices.If we are calling this assassination terrorism, then I completely believe terrorism is justified as an act of war.
Now for the "is it justified" part.... Justified means a lot of different things to a lot of different people. So let's boil it down a little...
1) Do you agree with it?
2) Would you pursue the people that did and aggressively hunt them down?
3) How much effort would you expend on their extermination, given what they have done (they committed an act of asymetric warfare and took out the Russian ambassador) . I'm sure that whomever was behind this had their reasons.
I think that a lot of the reaction to this may reflect how one feels about Putin and the Russians?
(e.g. if you've bought in to the notion that the Russians meddled in our election, and are pissed about it to some degree, then you might not care as much when their ambassador gets taken out.)
I for one - can see how the assassin may have had reasons, but I also think that there's going to be hell to pay from both the Russian side and Turkey and others over this.