Redstang281, it hurts me to see you continually turn a blind eye on some of the facts presented others. Therefore, I'm going to make this as simple as possible.
Let us suppose that there is a population of species A that exist in an environment. This is a predatory species where the individuals leave each other alone. However, through random mutation, an individual male of species A possess the instinct to actively kill young cubs and mate with the female species A. We will call him Bob. This mutation in his genetic makeup gives him an advantage over all the other species A members, because while they do not actively seek out his young to kill, he actively seeks out their young and kill them.
This ultimately leads to more individuals that are descendants of Bob. After many generations during which the males that most actively kill other males' cubs have more chances to pass on his genes through his young without much competition for resources. After many generations with the pressence of such selective pressure, a new species emerges (species B) where the male individuals of species B actively seek out other males' cubs to kill.
This is where "survival of the fittest" comes into play. In other words, the lions aren't thinking that if they kill others' cubs, then they'll pass on their genes more easily. Through millions of years of selective pressure, only the individuals with the most agressive instinct survive.
Feel free to nitpick everything I just said. I'll be glad to answer all your questions.
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