Responses
Yeap, in the case of lions, the reason for killing the old cubs is to put the female into heat. The female only ever goes in heat when she has no kids of her own, so when a male lion wins over another pride , his first task is to kill all the cubs from all the females within the pride. It's quite a sad scene because the females are also the breadwinners that go out to hunt and bring food back. So they not only lose the cubs, they also get knocked up, and feed the killer after all that.
I knew it. Toshimaru is in danger. He's planning to abandon the kid. That's what animals do when their primary mates have cubs or babies from other gene pools. They will kill off all the ones that aren't theirs due to the threat.