Simon's knowledge of the true beast is coupled with his knowledge of the boys themselves. He probably understands that trying to explain to them what the real beast is will only bring himself under further scrutiny and isolation. He's already regarded as odd because of his "mystic-ness" and previous inability to speak at assemblies. We've all had students in class that really don't like to speak in front of other people. As a result, the only time they tend to speak up is when they know their comments are dead-on and will be accepted by everyone in the room. Those students don't like to be placed under the microscope and forced to defend their own comments (which would, of course, lead to further talking). This is the group I've always though Simon falls under.
The littleuns are afraid of the beast, and Jack and Ralph are arguing about how to handle their fears Simon is tempted several times to speak up, but he is hesitant to do so because he has an inability to express his ideas to the others in a coherent manner, and he is well aware of this shortcoming.
Simon is truly terrified by what he has identified as the true "beast". The fictitious beast the littleuns are worried about is not the most worrisome issue on the island. Simon knows that they themselves ar to be feared above all else.
The dark side of human nature is to be feared because Simon realizes fear, the lack of structure and rule, as well as spiritual guidance will lead to destructive and malicious behavior. Simon is incapable of communicating this to the other boys.