The boys are to blame, yes. When we look at the choices that some boys made to remain civilized, we can see that those who became violent and savage had other options. They did not need to kill and hunt. There was fruit to eat. The violent path was a chosen one, not a necessary one.
The boys are most definitely to blame. As pohnpei397 states, they are the ones who made their choices and had to live with the consequences. The most salient reason I don't let them off the hook is that they do know right from wrong and civilized behavior from savagery, but they choose to drift away from what they know to be right. At the start of the novel they ALL behaved as they "should" have, but it doesn't take long for small and then larger crimes to take place. Through it all, there is a sense of shame/guilt in many of the characters evidenced by their painting their faces and acting as a mob rather than as individuals.
In one sense, the boys are certainly to blame. They are the ones who chose to act in such brutal ways once they were freed from adult supervision. No one forced them to do this and so it was their own fault.
On the other hand, you can argue that it is not really their fault. Golding is trying to point out that human nature is inherently brutal and evil. He is saying that people will be horrible if not kept in check by society. In this view, the boys are simply acting according to human nature. Blaming them for acting the way they do is like blaming a cat for playing with a mouse that it has caught. It seems cruel and terrible, but it is just the cat's nature. It's the same with the boys.
Expressly written as a response to another British novel, The Coral Island, a Victorian narrative about British boys stranded on an island inhabited by savages in which the boys effectively outwit and defeat natives in a victory of civilization over savagery, Golding's narrative clearly does not present anything like the concept of the noble savage of John Jacques Rousseau; that is, that man on his own is good, and if anything, it is society that corrupts him. On the contrary, Golding's plot records the degeneration of the boys to savages as a result of the adults', who represent the controls of society, being absent. This concept is first introduced in Chapter Four when Maurice joins Roger in kicking the sand castles that the small boys have made.
...Percival began to whimper with an eyeful of sand and Maurice hurried away. In his other life Maurice had received chastisement for filling a younger eye with sand. Now, though there was no parent to let fall a heavy hand, Maurice still felt the unease of wrongdoing. At the back of his mind formed the uncertain outlines of an excuse. He muttered something about a swim and broke into a trot.
Roger, too, without the constraints of society upon him is tempted to perform forbidden actions, for his intrinsic nature is sadistic. As little Henry plays by the seashore
Roger stooped, picked up a stone, aimed and threw it at Henry--threw it to miss. The stone, that token of preposterous time, bounced five yards to Henry's right and fell in the water. Roger gathered a handful of stones and began to throw them. Yet there was a space round Henry, perhaps six yeards in diameter, into which he dare not throw. Here, invisible yet strong, was the taboo of the old life. Round the squatting child was the protection of parents and school and policemen and the law. Roger's arm was conditioned by a civilization that knew nothing of him and was in ruins.
Later, with the erosion of this conditioning, Roger easily descends to the sadistic savage that rolls the pink granite boulder onto the rational, adult-looking Piggy.
The longer that the boys remain on the island away from society and its controls, Ralph's attempts to maintain and restore order are met with laughter, arguments, shouting, and brutality. At the end of Chapter Five, he longs for the presence of adults, the return to society:
"We're all drifting and things are going rotten. At home there was always a grownup....
"Grownups know things," said Piggy.....They'd meet and have tea and discuss. Then things 'ud be all right---"
When the intuitive Simon, who has experienced the lord of the flies, returns to tell the boys that the evil lies within them, the boys scoff at his attempts; later, Jack and his hunters are responsible for his brutal death.
Removed from society more and more, the boys grow dirtier and increasingly slovenly and primitive in their behavior. Moreover, the hunters paint their faces and beat others, finally killing Simon and Piggy; then, the hunters in a savage rage set fire to the island in their attempt to flush out Ralph and slay him. After Ralph is rescued, he cries for "the darkness of man's heart," the inherent evil that is in man, which, without the restraints of society, makes man a savage. Indeed, the boys are to blame for the destruction and chaos on the island, as they give vent to their intrinsic savage nature.
They were definately not because the guys that found them thought they had a sychotic breakdown from being seperated from adults, in which they probably would have been put through lots and lots of therapy to help get through the memories of that island.