• Because evil must be alone, as it cannot be trusted.
• Because your plans will not all succeed, and it is more harmful to be revealed as everyone's enemy than as someone's friend.
• Because caring about other people provides an additional source of motivation.

(I know ego depletion causes a reduction in acts of altruism, but I thought I remembered that engaging in acts of altruism could counter ego depletion. Now I can't seem to find any research supporting this, so perhaps not.)