Exchanging a few moves, Nero fought the male off, going so far as to heavily wound the male's foreleg, rendering it useless, and warned him then, "Get out of here, if you want me to spare your life." Her voice, then, was terrifying and, undoubtedly, she meant it.
She was only showing mercy because she didn't want to end his life in front of the child, even though he deserved to lose his life for being so willing to take the lives of others.