Why does it sound bad? Civilian casualties are civilian casualties, and attempts to justify them only lead to more casualties. As long as people on both sides support their governments, despite the civilian casualties, the violence will continue from generation to generation.
(And yes, Hamas is a terrorist organization, but it's also the legitimate government of Gaza. It may not be the de jure government, because the Palestinian National Authority ceased to have one in the aftermath of Hamas winning the 2006 elections.)
Civilian casualties always suck, but total war means they are unavoidable. Think of how many Germans were killed during WW2 just because Hitler wanted to take over Europe? War is never clean, and insurgencies will always use civilians as human shields, so civilian casualties are unavoidable if you do anything but surrender.
Also, if you don’t want to be invaded, don’t elect leaders that think kidnapping people across the border is a good idea. Ya, it’s rarely this simple, since these places are rarely so democratic, but people have some responsibility for their governments even in the case of autocracies.
Civilian casualties may be unavoidable, but a necessary evil is still evil. And, because I'm an outsider who doesn't identify with either party, I don't see the difference between Israeli and Palestinian casualties. Civilian casualties are civilian casualties.
Justifications similar to yours have been used to support the actions of Hamas. Many Palestinians, maybe even most of them, genuinely believe so. If the same arguments can be used in good faith to support both sides, such arguments must be worthless. They can only lead to more violence and more deaths.
WW2 was a rare war, because it was so conclusive. The Allies didn't only defeat the Axis, but they also had a plan for the aftermath. A plan how to reorganize the states on the losing side and how to win over the people and make them support the new regime. Without such a plan, a war today can only lead to another war tomorrow.
If the choice is between your own people getting kidnapped and killed by Hamas periodically, and total war, most countries will choose total war. Gaza will simply never be able to survive if its goal is the eradication of Israel, because Israel will simply not roll over and take that. Which country would?
(And yes, Hamas is a terrorist organization, but it's also the legitimate government of Gaza. It may not be the de jure government, because the Palestinian National Authority ceased to have one in the aftermath of Hamas winning the 2006 elections.)