In the critical essay "Secret Parables," V.S. Pritchett writes that with Golding's novel "We see how much a man is enclosed by his own eyes."  That is, in the struggle for self-preservation and survival certain emotions enter such as blind-love for one's own kind, hatred, and fear. These are the emotions that drive humans in the most elemental of conditions.  These emotions, then, are what cause people to conform, hoping that by joining others, they can gain strength.

Blind-love for one's own kind

Ralph and Simon are both altruistic, essaying to help the others by urging them to be organized, build shelters, and not let their fears overtake them.  Along with Piggy, these boys have a sense of community and strive to preserve civilized order and a protective environment.  They make every effort to dispel superstitious beliefs and fears.  But they fail through their conformity.

Hatred

Boys like Roger are elementally sadistic.  His concept of self-preservation is the elimination of anyone who conflicts with his intentions.  He and the hunters will survive, he feels, if others who disagree with his perspective are killed and out of the way.

Fear

Jack becomes the leader of the hunters and, later, other boys mainly through intimidation.  After they are beaten, Sam and Eric join the hunters out of fear for their lives. 

Conformity is a weakness, certainly, but it is also an evil. Simon knows this as he tries to tell the boys that the evil is "only us." It is an evil born of fear, hatred, and blind struggle for self-survival.  It is an evil because it excuses death for some fabricated reason.