They watched and did nothing for years was because of the syrian occupation which was backed by the USA(until recently). A little information for you, the christian lebanese militias were disarmed(many of the soldiers disappeared), there were two main christian leaders at the time, one was exiled and one was sent to jail(solitary confinement) for life. Others were also exiled and sentenced to death in absentia but they werent as powerful as the first two leaders. So just to recap, christians lose their weapons and their leaders, the christian party called the lebanese forces which rose from the christian militia was banned. If any demonstrations took to the streets(and believe me there were many) you would get half the syrian army coming to kick your ass and take you to jail.
All this time we ahve hezbollah happily getting weapons and growing stronger and getting more support because they are attacking the israelis in the south of lebanon. Then the only christians left(many of them also shiites), SLA, were left behind by the israeli pullout and were forced to either go to jail in lebanon or take refuge in israel(which thankfully israel accepted them).
All this happens and not a peep from the international community.
So please tell me how in the world are you supposed to say anything about hezbollahs weapons when they have them pointed at you and you have nothing.
The syrians left last year, and there have been many efforts from lebanon to disarm hezbollah politically because this is the only way that lebanon can disarm them. The lebanese military would in no way be able to face hezbollah because it is largely made up of shiites and it would break the military into factions(like the civil war) because they would not want to attack their fellow shiites.
So the only way lebanon could disarm hezbollah is politically and not forcefully because then a civil war would be sure to occur and we would have no weapons and would have to rely on the international community who conveniently have let us down in the past.
The Syrian occupation was never backed by the USA, I don't know who told you that but it is simply not true. In any event, the non-Hezbollah Lebanese had an obligation to resist the takeover of Lebanon by these foreign forces, by guerilla action of necessary. By failing to engage in such resistance, the silence equals consent rule applies. In order to not take a risk themselves, the non-Hez Lebanese simply transferred the risk over to Israel. Now that Israel has done the job the Lebanese should have done long ago, there's no valid complaint about the way they chose to go about it.