Caroline saw the vampire try to surrender, and then saw the shapeshifter kill him anyway. She didn't like it but there wasn't anything to do right now but accept it. Better safe than sorry, right?

The next vampire to try and escape didn't offer any pleas for leniency, which made it easier for Caroline to drive a stake into her heart. But it still felt a bit different, less of a necessary evil and more of a mindless slaughter.