The were the ones slaughtering his minions is the first thing that comes to Klaus' mind when he sees Stefan and Bonnie slaying the group of vampires that work for him. For some reason the younger Salvatore and the Bennett witch have teamed up. As a result the number of vampires being killed has doubled over the last several months. He doesn't know what it is but, if they were a nuisance to him individually, it's ten times worse now that they have combined their efforts.
Klaus stays hidden watching as Bonnie sets one vampire on fire and gives a fatal aneurysm to another. Stefan opts to use a wooden stake and if needed his bare hands. Klaus does have to admit they work very well together. They move in sync almost as if they are choreographing a dance sequence. Stefan uses a brute strength that is attuned to Bonnie's quiet strength. If they weren't screwing up his plans, it would be a beautiful sight.
Stefan finishes up with the last vampire and immediately hauls Bonnie into his arms hugging her tightly. Klaus rolls his eyes because it is not surprising to see Stefan act so syrupy when he's in white-hat mode. He is however surprised when Stefan pulls back from the hug to drag Bonnie into a fiery kiss that she responds to very eagerly. It's not the kiss itself that surprises him, what surprises him is the hot frantic passion that comes with the kiss.
He studies the way Bonnie grasps the ends of Stefan's jacket bringing him closer. He examines the way Stefan presses her against the tree behind them. He hears the moans coming from them and he comes to the realization that the two of them get off on the adrenaline of the fight. He doesn't think they are aware of it, they probably just think its relief that they came out of the fight unarmed. He knows better and he realizes that this makes them even more dangerous because it means they have more to fight for. He continues to look on as the kiss morphs into a slower, but not any less passionate kiss. This means that he will have to regroup and change his approach because no way will these two do-gooders defeat him.