Because our best interest is not the same as evolution's interest.
We, or, at least, some of us, have figured out, after a long time, that violence is not a good thing. But violence is what evolution runs on, because it wants the weak to die and the strong to reproduce. Altruism happened because groups of people ended up stronger than individual people, so there needed to be mechanisms to override violence impulses to enable groups. But the groups of people were still supposed to kill each other over and over to make stronger groups of people. The altruism is not universal. Hence all the ingroup biases. And much of that still remains to this day. All of us are violent directly or by proxy towards animals. Even if people are not violent, many are mean, they create pecking orders, they measure status, they compete, etc. They are executing adaptations developed to categorize people or groups of people as "should not be selected". The old programming is still there.
Except our combination of intelligence, empathy, sympathy, altruism, and strength "backfired" and resulted in us slowly deciding, for ourselves, that maybe we don't want to live like that.
I believe Christianity was designed to oppose this programming (original sin), but it's a 2k old religion, what do you expect? So much has changed since then, our methods in many places got better; Christianity is out of date and has too many dependencies.
I also don't think your characterization is fair, anyway. Christianity doesn't say we can only be bad or only good, it says that goodness demands complete purity, and through that, it says we're all bad, due to an influence inside of us, as well as an influence outside. While there are people more prone to violence and those more prone to love, there's nobody completely free from the programming or the situation.