From a biological point of view, the individual's decision to reproduce is simply about maximizing fitness. The individual is in competition with other individuals. The group is only important in so far as it provides a vehicle for maximizing individual fitness. This is not to deny the complexity through which the group can serve as this vehicle, but the group itself is unimportant. Natural selection has nothing to do with energy efficiency.
Natural selection picks the way that maximizes individual fitness.
There is no reason this has to be the most energy efficient way. If organisms coexist peacefully, they can make much more 'efficient' use of their resources. However, if one organism decides to use some of its resources to beat up on its competition, then it increases its fitness relative to the other. Repeated over time, this selects for the individual that beats up on its competition. In the end you end up with a group of individuals all beating up on eachother, wasting tremendous amounts of energy relative to the individuals that were able to cooperate.