I am not sure that Jack ever saw himself as the source of the evil, but it certainly is true that he uses the idea of the beast perhaps in this case even to construct a sort of religion around it, to absolve the boys of their responsibility and chalk the whole thing up to the decision of a malevolent god of some sort.
But I think that Golding also made this statement in order to make a comment about religion as a whole, that it can often be used to justify things or to assauge fear or to absolve oneself of guilt just as Jack uses it. The fact that a young man is able to put together this "theological speculation" and use it to his advantage and to maintain his power can also be seen as a commentary on religion and religious leaders.
In order for Jack to maintain his rule over the boys he must present the killing of Simon as the will of another force rather than the result of an internal evil that resides in the boys responsible for the murder.
Where Ralph is unable to end his followers’ misery over Simon’s death, Jack is able to relieve his followers’ doubts with theological speculation. Ralph fails with the truth, while Jack covers the truth by shifting the blame to the beast.
The speculation that the cause of Simon's death is due to the beast or some other divine happening rather than the boys places the blame on outside forces. This implies that Jack no longer see's himself as the source of evil.