This is a good question. I would like to say that Simon prevents evil, but he does not in the end. In fact, as you know, he is killed. This shows a rather bleak picture of humanity.
Throughout the novel, there is a declension of human morality; things get worse and worse as time progresses. At first the boys are civil because they still remember their old world with its rules, laws, and customs. However, as time progresses, the boys realize that a rescue is not forthcoming. Little by little the savagery of the boys emerges. Jack is the essence of this savagery. Ralph, who represents civilization, tries to moderate things, but he is eventually eclipsed and runs for his life. I would say that the boy who is diametrically opposed to Jack is Simon. But Simon could not stop violence. His voice was not strong enough.
That said, he did help in one important way. He had the great realization that there was no beast. Instead, the beast was within. He says:
“Hear him! He’s got the conch!”
“What I mean is. . . maybe it’s only us.”
That was from Piggy, shocked out of decorum. Simon went on.
“We could be sort of. . . ”
Simon became inarticulate in his effort to express mankind’s essential illness. Inspiration came to him.
In short, Simon could not stop the violence, but at least he saw where it came from.