Though she certainly was glad that he wasn't killed, Hermione thought it strange too. Most Death Eaters killed without thinking or caring much about the ramifications. A lot of changes had taken place in their world since her husband murdered the Dark Lord with a pillow, but not that much. Only someone acting on orders to not kill Thorfinn would've bothered to spare him. It was the smart move to get rid of Thorfinn as soon as possible. Clearly their attackers knew them well enough to know that fact.