Liquid genuinely wasn't expecting him to actually go for it, so he seems a little surprised. The thirst, though, urges him to go onward. It's hard to stop himself from jumping on him immediately, but he takes a moment to speak.

"I wasn't planning on killing you." Not that he was planning on anything but it's probably better to tell him that than anything else, isn't it? "I'll... explain later. Just..."

Part of his mind takes in that whole 'friends' part, but that's kind of shut down when his instincts take over again and he once again sinks fangs into Mettaton's neck.

Don't worry, he's sure he'll take control again before he can kill him.