I am by no means an animal activist, and there is too much human suffering and infanticide to even think about spending time on animals, but I have often struggled when thinking about this.
There is, I feel, a link between the two. Let me say first that sin of infanticide is obviously worse than that of animal cruelty, and that a child is of immeasurably greater worth than an animal. That said, both animal and child are given to the dominion of man, the former universally, the latter in the person of its parents. The road to rationalizing infanticide, or any unlawful killing, is so often first taken in animal cruelty. When one can justify, or worse still take pleasure in, the mistreatment or cruel death of something that is without malice and given into one's protection, the very notion of the sanctity of life and the demand of corporal mercy are degraded and ultimately disregarded.