I've asssasinated plenty of assassins. Egypt seems to train them when they suspect sabotor against them.

The trick is to have an assassin survive a few misions once he has then you have a deadly weapon on your hands. I've had an asassin complete more missions than about 10 others that I had trained.

This one asassin in his lifetime killed 6 Egyptian family members, killed 5 assasins and damaged maybe about 30 Egyptian buildings throwing many of the cities into revolt and yet the Egyptians still managed to survive.

Another nice little tip (have'nt done it yet) but when asassins/spies are far away you can throw them on a ship to transfer retinues