Downplayed: Bob isn't a sociopath who takes matters into his own hands, but supports legalizing Euthanasia and promotes birth control not out of compassion but because of "population control" and "for the good of society as a whole".
Bob constantly talks about how stronger groups survive and weaker groups dissolve, but reveals that he isn't talking about warmongering dog-eat-dog-ism, but the benefits of friendship and mutual cooperation.
But he still acts like a Social Darwinist around people because he's a bit of a jerk, and he can.
Bob eventually goes He Who Fights Monsters, and eventually dominates and tortures said Ill Girl, reducing her into a mere pet and plaything, because in his eyes she's too weak and pathetic to be even considered worthy of being human.
Exploited: A Manipulative Bastard uses the Social Darwinist fad to recruit henchmen (particularly the fit and confident ones) for his purposes.
Bob, an evolutionary biologist who continuously writes about natural selection and survival of the fittest, gets accused as a Social Darwinist, but rejects the implication and encourages altruism.
Real-life evolutionary biologists are actually obsessed with maintaining biodiversity and preserving endangered species that, according to Social Darwinists, are supposed to be wiped out by Nature's Law.
Discussed: "Why does it have to be that the strong should dominate the weak?"
Conversed: "Social Darwinists. I hate these guys." "Why?" "First and foremost: Darwin does not work that way. They just give him a bad name."
Bob's social Darwinist policies cripples his faction's abilities to preform essential functions aside from making war, and possibly the ability to do that as well.
After losing many battles, Bob begins to see himself as inherently pathetic, weak and unworthy to his tastes, and is Driven to Suicide.
Bob's repulsive Social Darwinist behavior drives away everyone, and thus is even more prone to being wiped out of the gene pool, since in Real Life cooperation is more advantageous for long-term survival than individual strength.
The reason why Bob is a social darwinist is because he really had to fight/struggle hard to get what he wanted, which means that he's a Self-Made Man, but the shady game he was forced to play in order to reach his goals, and the hardships he faced through his struggle, has made him sour and cynical of the world in general.
Bob only uses social darwinism as an excuse to be a jerk and treat others like shit, and he really doesn't believe in the whole "hard work, strength and intelligence = success and life fulfillment" part of the social darwinist ideology. He only got what he wanted by fooling, luring and using other people who actually should be in charge due to their creativity or hard work, and making life difficult for those who saw through his bullshit.
After losing many battles, Bob begins to see himself as inherently pathetic, weak and unworthy to his tastes, but instead of being Driven to Suicide, he responds with "I should improve myself anyway."
Bob does realize that in real life cooperation is more important than individual strength, making him decide that being a Manipulative Bastard would have greater benefits.
Played For Drama: Bob is proven to be a hypocrite, using unnatural means to prolong his life and become stronger, rather than letting nature take its course.