Moroni tells us that he is writing about the fact of the secret combinations as a warning to us. The evil one may tempt us into believing that we too might murder for gain, and unfortunately we have a society where some form of murder for gain is rampant. Moroni is telling us that this is no long term solution, that it will be ultimately destructive. Certainly in the modern microcosm of violent gangs, the impetus to violence leads to very short life-spans for many of the gang members themselves.