I'm no Bible scholar, but doesn't that verse you referenced say something like "Then came the day of unleavened bread when the Passover lamb was to be sacrificed"?

Yes, we know the wider community in which Jesus and the disciples were living would have been sacrificing a lamb, (as they were to sacrifice Jesus later) and this would have been an important allegorical part of the story to mention. But nowhere in that verse does it say that Jesus instructed Peter to kill a lamb.

In fact, he simply tells Peter to go and make preparations so that they could eat. Given that the day is specifically referred to in that verse as "the day of unleavened bread" and not "the day of butchered flesh" I fail to see where you're drawing your conclusion from, especially given what I've said about the Essenes.