Even in wartime, assassination is tricky. The decision to shoot down Yamamoto's airplane, when his itinerary was discovered through breaking the Japanese code, was approved by President Roosevelt. No one at a lower level was willing to make the decision. They all bucked it higher.
However, assassination in peacetime is a lot more tricky. What happens if the plot is discovered before the attempt is made? What happens if the attempt is successful but the plot is broken afterwards? The nation whose leader was assassinated might well take it as an act of war, precipitating the war the assassination was supposed to prevent.
It's worth noting that Hitler survived at least five and maybe more attempted assassinations. All but two were either discovered ahead of time, or aborted because the assassins found Hitler was too well guarded. The two attempts that were actually carried out (the Beer Hall Bomb and the Generals' Bomb) failed for various reasons. Hitler is probably the purest example of what in retrospect would appear to be a justifiable assassination, yet the difficulty of carrying it out provides a useful lesson.
Another lesson is that of the successful assassination of Reinhardt Heydrich by a group of Czech commandos. He was replaced by someone who was even worse, and several thousand people were executed or (in the case of Jews) "sent East" as a result of the successfull assassination. The village of Lidice was wiped out completely, and everyone in it executed.
All these things need to be taken into account when someone talks glibly of assassinating a foreign leader.