It's the only way women knew how to protect themselves. Throughout history, women never had an active role in much-- they were objects, not subjects of the world. Simply an extension of men. Men have always had a physical advantage (what allows them to be the dominant gender), and warfare is no different-- in fact, that limited female reality is magnified x1000. We're talking about women who've been
captured by the enemy (thanks to the failure of their men). There's not much room for negotiation here if we're honest. Women could either become "spoils of war", akin to resources, and
survive...or they could be "loyal" to the dead, and
join them in the grave. The options were bleak.
This summarizes what I'm saying a bit better:
https://www.springer.com/gp/about-s...es-highlight-how-ancient-women-survived/40742
As for why lionesses would allow their young to be killed by the new male...I don't quite understand that from an evolutionary perspective. Usually females are evolved to be extremely protective over their children, even sacrificing her own life for the betterment of theirs. A woman's first loyalty is to her kids.