I broadly agree that babies aren't people, but I still think infanticide should be illegal, simply because killing begets insensitivity to killing. I know this has the sound of a slippery slope argument, but there is evidence that desire for sadism in most people is low, and increases as they commit sadistic acts, and that people feel similarly about murder.

From The Better Angels of Our Nature: "Serial killers too carry out their first murder with trepidation, distaste, and in its wake, disappointment: the experience had not been as arousing as it had been in their imaginations. But as time passes and their appetite is rewhetted, they find the next on easier and more gratifying, and then they escalate the cruelty to feed what turns into an addiction."

Similarly, cathartic violence against non-person objects (http://en.wikipedia.org/wiki/Catharsis#Therapeutic_uses) can lead to further aggression in personal interactions.

I don't think we want to encourage or allow killing of anything anywhere near as close to people as babies. The psychological effects on people who kill their own children and on a society that views the killing of babies as good are too potentially terrible. Without actual data, I can say I would never want to live in a society that valued people as little as Sparta did.