That has sharpened fears the 30-month-old bloodbath in Syria is steadily encroaching on Lebanon, still haunted by the slaughter of its own civil war (1975-90) in which the Syrians played a murderous role.
These concerns have been accentuated because the Syrian conflict has become a sectarian killing ground, a war less about overthrowing a brutal regime and more about the religious-ideological confrontation between Sunni Saudi Arabia and Shiite Iran.
It is unknown who was behind the killing of Sheik Saadeddine Ghiyyeh, shot to death by masked gunmen on a motorcycle Tuesday.
But it's generally believed he was slain by Sunni hardliners who saw him as a traitor to his sect at a time when Hezbollah is increasingly dominating Lebanon, threatening to overwhelm the Sunni and Christian merchant and political class that has run the country since independence from France in 1943.
Hassan Nasrallah, Hezbollah's charismatic leader, warned Tuesday the assassination "is an indication of a dangerous path that has started in Tripoli and could reach other regions of Lebanon. There's a concern that new directions are being taken in Lebanon."
Ghiyyeh was a top official with the Islamic Action Front which is allied to the Syrian regime of President Bashar Assad and has close relations to Hezbollah.
Syria's Sunni majority and foreign jihadists are spearheading the rebellion to topple Assad, a member of Syria's minority Alawite sect, an offshoot of Shiite Islam.
Hezbollah, at the urging of its paymasters in Tehran, has sent thousands of its best fighters to support Assad. They've become his shock troops.
Hezbollah's intervention defies the Lebanese government's 2012 policy of "disassociation" in the Syrian war. But that held little water after Hezbollah brought down the government in March, leaving the country adrift.
Tripoli has become the barometer of sectarian rivalries in Lebanon because of persistent battles between pro-Assad Alawites and majority Sunnis who seek Assad's downfall.
Dozens of people have been killed there. There's been trouble on a smaller scale in the southern city of Sidon, long a Sunni bastion and the place where the first shots in the 1975-90 civil war were fired.
Hezbollah's Dahiyeh stronghold in south Beirut was hit by several car bombings, killing and wounding dozens of civilians, after a series of bombings against Sunnis in Tripoli.
The worst was an attack on two mosques Aug. 23 that killed 50 people and wounded 350.
And there are constant clashes and killings along Lebanon's northeastern border. Both sides use the region as supply routes into Syria.
This is an area dominated by clans of both sects, hard men deeply involved in smuggling, including narcotics from the Hezbollah-controlled Bekaa Valley.
They're pretty much a law unto themselves and they believe in an eye for an eye.
Amid this tribal warfare, there's a battle looming and it could ignite the smoldering, centuries-old sectarian hatreds that are part of this rugged land.
The Assad regime's building up an offensive to take the strategic Qalamoun region which runs from north of Damascus all the way west to the Bekaa, Hezbollah's heartland. If the regime succeeds, its survival is pretty much assured.
Hezbollah, under Nasrallah, has sought to avoid a head-on clash with Sunnis or al-Qaida in Lebanon because it can't afford a fight on two fronts.
But it may come to that, possibly with the Saudis stirring things up in Lebanon to force Hezbollah to pull back its fighters from Syria, where crushing Assad, Iran's ally, is Riyadh's overriding objective.
There are persistent reports Hezbollah is discreetly withdrawing from Syria, possibly because it sees trouble brewing in Lebanon.
Hezbollah, typically, has no comment on that. But analyst Hussein Ibish maintains Lebanon "is facing the prospect of unprecedented state disintegration" because of the Syrian war.
"The reality is that Syria is starting to look increasingly like Lebanon: fragmented, splintered and ruled in fact by different groups in their discreet areas," he observed.
"The irony is that Syria's transition into a Lebanese-like reality may destroy the ability of Lebanon to maintain its own uneasy equilibrium."