What is the ethical and/or moral position regarding such action?
Interesting question - does it come under the same heading as attempting assassination of Hitler to end the war early - or that of arranging the death of a rebel-rouser to prevent the Romans cracking down on the Jews in AD33?
I would say that the answer is that he should not be offered the deal, but should be arrested by the Yemeni police/army! The trouble with this line is that the repression would go on until a coup d'etat forced him from office, which could be a while if the army/police are not sure enough of their support or strong enough without support - and would the latter be any better? - Though equally will his vice president be any better - after all he is a (presumably) trusted member of the ruling regime, and at that level will be heavily implicated in the repression that has been happening.
Of course, the immunity from prosecution is limited to those who sign the agreement, not the international community, so he could still be tried at the International Court....