The United States waged war on Afghanistan and Iraq after terrorists killed so many innocent civilians on 9/11. Israel waged war on Hamas after rockets landed deep within its borders.
But despite evildoers (those who would kill innocent civilians) provoking the retaliation they deserve and over time in the court of global public opinion, more and more people see America and now probably Israel as the wrongful aggressors for beating up on much smaller, weaker foes.
One explanation may be that good vs. evil is a matter of interpretation. And it may be that just like beauty, evil is in the eye of the beholder. From the P.O.V. of Islamic terrorists a very distorted picture of America as the greedy, oil gabbing infidels can be painted clearly enough so as to be believed. And of course it takes no imagination for Americans to view the Osama bin Laden and the Taliban as perpetrators of evil bent on destroying America and all Americans. Similarly it takes little for the Hamas and Israelis to paint similar evil portraits of each other.
On the other hand, big vs. small is more concrete and something you can wrap your hands around. Therefore after the initial terrorist attacks of 9/11 or rockets in Israel fade somewhat into the memory of all but those directly affected, what seizes people's attention is that a overwhelming force is fighting a much smaller and weaker one.
At that point, since the majority of people view themselves as small and oppressed by those in power, there is a great temptation to identify too much with the Islamic fundamentalists and Hamas and react against the much stronger Americans and Israel based merely on the fact that they are much smaller.
For larger powers like America and Israel to say, "But they started it," seems almost as if we "protest too much" and doesn't seem to have much effect in dissuading those who root for the underdog to stop doing so.
I don't have an answer for this tendency of human nature to believe the concrete (big vs. small) over the abstract (good vs. evil) other than to have the victims and their families keep telling their stories* so that people don't forget who the bad guys truly are.
Victims of the holocaust have done just that and although the immediacy of that event is fading into history as the last survivors die off, it has kept that awful memory alive for sixty years.
* While I'm on the topic of stories, there is much more to telling one than a plot and characters and two great resources to finding out more are Peter Guber, Hollywood uber producer and icon and Stephen Demming, the brand in business story telling.