For those of you who don't have the same terrifying vocabulary that I do, infanticide is the practice of killing children. Specifically one's own children. Bears and lions are the worst offenders of the mammal world, often killing the offspring of other males to prevent competition. Whenever a new male lion takes command of a pride, his first order of business is to pick off all the existing cubs, because they're the previous leader's children, not his. Females won't go into heat if they're already taking care of cubs, so his evil plan is essentially to get to mating as soon as possible. Bears aren't even this organized. A male bear will attack any cubs he meets as long as they're vulnerable, even if there's a distinct prospect that they're his own children. To him, all other bears are direct rivals for food and territory. He sees baby killing as a preventative measure, taking care of the problem before they get big enough to be a threat to him. That's why mother bears are so famously protective; leaving their kids alone for too long could mean letting them be murdered for no particular reason.
This isn't a feminist tirade on why females are universally better parents than males, mind you. In the spirit of gender equality, I'd like to make note that there's plenty of infanticidal mommies in the animal kingdom too. There's no birth control in the natural world, and when times are lean, kids are the first liability to go. Stressed rabbits and pigs will turn on their offspring and eat them, taking back some of the precious calories and nutrients they sacrificed on the litter. Fish and insects are less affectionate than that: in many species, the minute you're out of the egg sac is the minute you switch from apple of her eye to, well, dinner. Anyone who's ever kept a tank of guppies will tell you that a good three dozen fry can become eight or nine if the adults get a hold of them, and mum's just as much of a candidate as anyone else.