Ah the evil dialectic of exigent capitalist politics! Megrahi, obviously innocent of blowing up an airplane over Lockerbie, Scotland, is sacrificed by his government, at the insistence of the UK and US, to get sanctions lifted. Carriles, who boasts of blowing up a Cuban airliner, is protected by the US government.
http://www.guardian.co.uk/world/2011/apr/10/luis-posada-cuba-denounces-us-acquittal-cia-agent