There is, unfortunately, a very long list:

https://wikispooks.com/wiki/US/Foreign_Assassinations_since_...

That site doesn't seem very trustworthy.

What site does these days?

I mean, does:

http://www.theweek.co.uk/politics/21051/cia-and-long-history...

.. or:

http://www.globalresearch.ca/the-cia-has-attempted-to-assass...

.. or:

https://www.theguardian.com/world/2003/mar/21/usa.davidpalli...

.. seem more or less trust-worthy to you? Perhaps there is a site that lists all the people America has not assassinated ..

Well, someone succeeded with JFK...

Lone nut-cases are hard to predict, therefore hard to stop.

a domestic enemY is easier to get rid of than a foreign one.

I'd think that assassinating any human being is particularly easy. They are flimsy creatures.

Unless Castro is living secluded in a bunker all day with only 3 servants, he'd be seeing hundreds of other humans everyday, any one of which could kill him.

Maybe, though you might be able to say the same about the president of the US. The people surrounding the leader of any given country I'd guess would be people who have background and links, kinda like a network of trust (I'm thinking GPG here), one you have to infiltrate further away from the target? Families and relations and ties etc. Guessing most political leaders don't hire staff from temp agencies?

That we know of. Which may be exactly the way they want it, and that may add to the difficulty.