My conspiracy theory wasn't really aimed at Syria, PJ. I was going just a little more tinfoil--militants killing militants. You've got a guy that's been in hiding for over a decade. Even though he's pretty high on the food chain, it's got to hurt his effectiveness to stay firmly in the shadows. Sometimes your best gambit is to sacrifice your queen.
That's a possibility: if the rest of the Hezbollah leadership had decided that Moughniyeh had become a liability, this would be the maximum-value way to get rid of him. I have no way of knowing if this is the case, but going by what I know about Moughniyeh (i.e., that he's about as evil and psychopathic character as they come), we can't rule it out.
Recently, Hezbollah seems to have been marginalized a bit.
A bit, yes. The March 14 movement has certainly gotten more vocal in talking against it. But, again AFAICT, there's been no fundamental power shift there -- they're still the most effective fighting force in Lebanon, and, if push comes to shove, they will command the near-unbroken support of the biggest ethnic/religious group in Lebanon. IOW, (AFAICT), they're not on the brink of collapse or anything.
Similarly, more Arab nations are playing nice with Israel these days than usual. Even though the peace talks won't amount to much, the fact that both sides are even acknowledging there's someone on the other side of the table would have to be considered progress. What better way to put a monkey in the wrench than to pin a prominent assassination on "the Zionists"? No peace, a good "flag wave" for Hezbollah's recruiters, and a solid step back to more traditional power structures.
Except... the Hezbollah is a bit player on the Israel/Arab stage. If Israel and Syria sort out their differences over Golan, and if there is genuine progress on the Israeli/Palestinian front, the Hezb won't even be invited to the discussions. They're an immensely powerful player inside Lebanon, they have the capability to put the hurt on Israel (at huge cost to themselves), but internationally they don't really count for much.
Moughniyeh's assassination will further destabilize the situation inside Lebanon, but the Hezzies don't need any help in recruiting (the Lebanese Shi'ites are already behind them) and I can't see how they could *possibly* hate the Zionists more than they already do. Having your homes razed to the ground a couple of times will do that far more effectively than having some guy in Damascus blown up that you've only seen in pictures.
That said, I don't know that it would be a major surprise to me if Israel really did do it.
As stated, I think it's the most likely solution. Another possibility is that Nasrallah and Fadlallah decided that he'd become a liability and decided to cash him in... but, off the top of my head, I can't recall them doing anything like that before, except to people who actually turned traitor, naturally. These guys are pretty damn loyal to each other, and it would take a lot to break that. But, then again, they're also cool-headed and think strategically, so if they decided there was more to be gained by getting rid of him... who knows?