Targeted assassinations can be justified when the target is a legitimate enemy who is actively engaged in planning or organizing or carrying out criminal or terrorist activities, and when it's possible to hit the target without killing innocent people. Also, when it's not possible to bring the targeted person to justice in a normal way; when he isn't living in a zone of peace where law and order prevails and policemen make arrests, but when he is living in something more like a zone of war. When those conditions are met, I think this is a legitimate response to international terrorism.
I don't know a lot about Mabhouh. He seems to have been a legitimate target, but there are prudential arguments against hitting him in a country that has been in fact, without anybody acknowledging it, a strategic ally of Israel's -- a semi-friend. And using fake British passports -- I mean, it was done so badly that obviously it shouldn't have been done. The French have this saying: "It was worse than a crime; it was a mistake."
The United States is now committed to targeted assassination as a policy. We have done it sometimes successfully and sometimes with a lot of civilian deaths. It should be the policy of the United States in Afghanistan, and probably in Pakistan too, that after you carry out one of these raids, you should be prepared to defend it. You're using the coercive power of the state in a lethal way, and in a democracy -- in a country committed to the rule of law -- actions of that sort should be subject to some kind of public scrutiny.