This would be the easiest to do, but I think the repercussions of such an act would be big. They both are important people and more or less are internationally recognized representatives of their supporters.
Even in the harshest fights there are some rules which should not be crossed. In the case of Israel or the US the situation is different of course, because as we know the justice system established right now is working for the higher bidder and the guy with the biggest... rifle. As we know Turkiye is nowhere near these two in terms of bidding capacity and rifle length, Turkiye also don't have the international support which would at least soften the reaction. Highly secretive in our times is not so highly secretive after all and our enemies can also breach the rules and start hunting for our people around the world. This kind of intelligence war is extremely tiring and it will be sucking resources that could be used for something else.
In the case of Hafter it may work, but it is not a good thing to breach the etiquette as we know Hafter is connected to foreign intelligence services as well. In the case of Assad I don't think this would bring the wanted result. Syria is basically ruled by Assad family and the Muhabberat (military intelligence, air forces intelligence). It is a mafia State. For example they are one of the biggest producers and exporters of drugs in the Middle East right now. The State is controlled by many powerful people with big businesses and international connections. Even by some chance Assad dies, there will be another one that will take his place. It is an already established system.
If we put aside the international reaction we should carefully analyze the results and we should calculate the next moves because this will not end with their death. We should see who are the possible alternatives that would come to their places and if it would be better, worst and what is the possibility for exploitation. If such a thing didn't happen till now there is possibly a reason behind it. Hafter was actually close to his end...
BTW the Turkish Intelligence already has experience in making big booms in Syria from the times of Ocalan. Our guys are not worse, but better from these times