But consider what would happen in this country, if a terrorist organisation sprang up that consisted of Brits who started killing, Germans, for some weird political/religious reason. How long do you think Germany would tolerate that if we did absolutely nothing about it?
I am not saying that it is ever OK to specifically target civilians, but there comes a time when you just can't be too worried if there is collateral damage.
The terrorists specifically targetted civilians, and that is never acceptable. When Israel responds, well, it's impossible not to hit civilians, and in fact, it's pretty difficult to even tell who is a civilian and who isn't.