Quite simply: there are limits to how much harm and violence can be justified by appeal to "self-defence". Perhaps Israel would be safer if it nuked the rest of the Middle East out of existence. The plain impermissibility of such action indicates the need for more considered reflection on the moral status of specific Israeli retaliations than merely asserting, "I believe that Israel has the right to defend itself."
As RadGeek writes (in response to an IDF commander's claims that "we are here to show that if, God forbid, any of us is captured by the enemy, the army will do everything to secure his return"):
The murder of civilians by Palestinian or Lebanese terrorists is criminal, and those who committed the murders can be stopped from committing further crimes through the use of violence, if necessary. But the right to use force against someone does not mean the right to use any amount of force necessary against anyone at all in the process of stopping her. It’s true that if you really are willing todo everythingin retaliation for the kidnapping of a soldier, or attacks on your forces, or attacks on civilians, then this is included. Any atrocity at all is included indoing everything,and that is precisely why the willingness todo everythingin retaliation for an attack, no matter what the cost to innocent third parties, is a moral crime of the first order. Destroying the lives and livelihoods of scores of innocent people in the process of trying to stop the murder of one or two other innocents is criminal. Destroying the lives and livelihoods of scores of innocent people in the process of trying to avenge the death or capture of a handful of soldiers in combat — the primary justification given by the Israeli government for these campaigns — is nothing less than an atrocity.
Alonzo Fyfe insightfully argues that the right to "self-defence" is better understood, not in retributive terms, but more broadly as "a right (and perhaps a duty) specifically to defend the innocent from those who do harm to the innocent." He continues:
[W]hen we define the “right to self-defense” as Israel is seeking to define it [this leads to...] a state of perpetual war with competing sides both claiming the the right to kill the other in the name of defending their own.
We get something entirely different if we define the right of self-defense as “the right to protect the innocent from those who would do them harm.”
Not, “the innocent Israelis from those who would do them harm.” Not, “the innocent Americans from those who would do them harm.” But, “The innocent.” Period. Full stop. From those who would do them harm.
Applied to the current crisis:
On this standard, the deaths, injuries, and economic and psychological harm inflicted on many of the people in Lebanon are moral failures. Many of those being made to suffer are innocent. Under the principle, “Protect the innocent from those who would do them harm,” every bit of harm suffered by the innocent is a moral failing. It can never be defended as truly good. It can only be defended as a lesser evil.
Only, in this case, there isn't anything "lesser" about it.