I thought about intervening to save the rodent's life, but before I could get very far, I noticed it was already limping pretty badly, and I figured it might be better in the long run to have Pierre dispatch it.
This activity is one of the elements of cat life that I have a hard time with. It's not so much that they hunt other critters, it's that they play with their prey before killing it. Cats have a good time while another critter suffers - seems intentionally cruel.
Pierre sure did have a good time. Thankfully he took his prize elsewhere for the final finish. Rest in peace little rodent.