I believe a bombing campaign with the aim to toppling the Assad regime, similar to the campaign in Libya, is justified.
The primary reason is that allowing the civil war to continue would likely cost more lives then a bombing campaign would.
If we do not intervene the Syria civil war could end up much like the civil war in neighboring Lebanon, where the multi-side war lasted for more then ten years and didn’t really end until there was international intervention, by Syria. The Syrian civil war has cost 100,000 lives so far. If it continues for another 10 years the number could reach 500,000.
In Libya the bombing there cost relatively few civilian lives. During the bombing Toronto Star reporter Rosie DiManno was in Tripoli. She was kept under continuous control by the Gadhafi regime, never allowed to talk to people on the street without supervision. They tried to show her all the civilian causalities, but the best they could to come up with is a single body every day or so, and sometimes the doctors would take the reports aside and tell them the victim has died in a traffic accident or something.
A bombing campaign could be more difficult in Syria then other places where it has worked, like Libya and Serbia, because the Syrian military is more advanced. They could even lose fighter pilots to the enemy, but the number of western lives lost are likely to be tiny compared to Syrian causalities.
One of the major concerns about intervening is that many of the rebels are Islamic extremists. While this is true, I think it is a argument to intervene as soon as possible. It is the civil war that has brought the extremists to Syria. Allowing the war to continue will only make the more entrenched.
It would have been better to try and end the war earlier, but even now I doubt the extremists would have much support among the people of Syria. They may make up a significant percentage of the rebel fighters, but that doesn’t equate to popular support, especially since many of them are foreigners. However, the longer civilians suffer, the more popular support the extremists will get.
The only counties where extremists have gained a significant foothold are Afghanistan and Somalia, two countries that have been in a state of almost continuous war for decades. Keeping the Syria civil war short will help keep the extremists from gaining any more of a foot hold. If we are able to end the war, the extremists may still commit bombings and other terrorist attacks, but the level of killing will be much lower.
It should be possible to provide weapons and support to the secular rebels. This article argues that they are separated by geography and ideology and dislike each other almost as much as they dislike Assad.
A bombing campaign against the government forces to bring victory to the secular rebels would be a risk. There are a lot of things that could go wrong, but I believe the risk of allowing the civil war to continue is likely to cause more death in Syria and a greater chance that Islamic extremists could acquire chemical weapons.
Perhaps to greatest benefit is that it would send a message to other dictators that the world does not accept using massive military force against democracy protestors.