Kids in danger from Momma's boyfriends?

This doesn't apply to me personally, but I've read so many times over the years stories of children, especially little children, being murdered by their mother's boyfriends for no apparent reason. I can't help but think of a nature program I saw years ago about some lions, where the mother lion had to hide her cubs from a new mate after her 1st mate was killed. The new male lion would instinctively try to wipe out his predecessor's genetic line on favor of his own. By killing the cubs. I can't help but wonder if there is a similar instinct in humans. I can't see why not, it's just, as reasoning beings, we can fight off that primal urge and act civilized. Most of the time...