I don't think killing a foreign leader would be necessarily helpful. The president of Iran is really a puppet for the ruling clerics, in the background. Kill him, and they'll just make another guy president. South Korea...I think this guy has a son...etc. When leaders in this country have been killed (JFK), our system has continued on.
I would much prefer using black ops to make life difficult for existing leaders....and I am sure they are doing this. It would range from spreading rumors that they have VD, to paying off a scientist to sabotage uranium enrichment.