Why'd she slay her infant girl so brutally?
The Abenakis killed her infant...
It's what enemies do. Read some of the Biblical stories of wars and conquerings, "dashing your children against the rocks" was a dire warning to the Jews. OTOH, just check out your nearest Islamic-Jihadist training camp.
She didn't kill the baby, the captors did.
The baby was only a few weeks old, and was hungry. They wanted her to make it stop crying, but would not let her stop to feed the baby.
What happened to the captors was a result of them killing the baby.