Hariri, a pan-Arabist and Lebanese nationalist, was known to adamantly oppose the construction of a major U.S. air base in the north of Lebanon. The United States wants Syrian troops completely out of Lebanon before construction of the base is initiated. Hariri's meetings with Hezbollah shortly before his death also angered Washington and Jerusalem, according to the Lebanese intelligence sources.
It sounds rather ridiculous considering that Lahoud and the other pro-Syria politicians would be just as against such an airbase. Why not kill them too? Also, I've never heard of plans for an airbase in Lebanon, especially with the Incirlik base in Turkey is quite close.
Part of the problem with this "who benefits" analysis is that, as in any crime case, you can't just rely on motive. Opportunity and capability also have to come into it.