Israel is a recognized state and by virtue of its statehood and international recognition, it is its responsibility to protect human rights and act according to international laws. Alas, it isn't being forced to do so by international community and has been getting on with its high handedness on Palestine, thanks to the complex international relationships of interests and benefits.
On the other hand, Hamas is a recognized (by international community) terrorist organization which is not bound by international laws of human rights. They can commit crime against humanity (whether they actually do so or not is another matter for another discussion) because what they do is itself defined as crime. They are not morally obliged to stay clean of human right violation unless they are no more recognized as terrorist organization, recognized as valid representative of Palestine and its people, and hence are morally obliged to uphold common international human rights.
It is something like this. It's not a moral duty of a criminal to uphold laws because by virtue of its definition, it is a violator at the first place.
Similarly, it is not morally justifiable for police to violate laws, even against the violators, because by virtue of its definition, it is the protector of the laws.
Again on the same ground, it is morally indignant to kill a perceived terrorist without proper evidence and trial (exclusive of the instances where it is a question of life for their security forces). That would mean genocide of the very values forming the base of the state, especially if it is a democratic one.
In nutshell, all those movies you saw where the honest policeman takes the law in hand (I don't know how they do it as it ain't a physical object small enough to be held in human hands) and terminates the villain, foreign terrorist, and corrupt politicians with all blood and gory are simply morally unjustified.