Good or evil isn't the issue. The defining characteristic of humanity is laziness. The people who failed to help Dahmer, or to stop him earlier, did so because it was easier than doing the bit more that would have been required to actually achieve anything. They went through the motions, arrived at the quickest and easiest way to get the problem out of the way and went on to the next, to deal with it the same way.

Unfortunately for his victims, Dahmer was less lazy than most, and began to actively follow his urges. I hate to say it but there are problaby a great many people out there who would be serial killers, except that it's too much like hard work and they can't be arsed!