It is moral because if he was God - which whether or not you believe it the argument is being based of a book which proclaims it to be the truth - then any action he performs is inherently moral. It's internal logic. It is immune to any subjective debate.
He was the lamb that was given to be sacrificed to safe all human kind. He did not kill a lamb to save a human being. Where in the world this came from I have no clue. I googled to see if someone came up with this idea, but it is not showing up. So I am a little lost as the the question since this act was not recorded.
Of what I have heard of the bible, Jesus is all forgiving and loves all and so on. However, it is immoral in the first place that God gave humans authority over animals, and even more so that he killed a innocent lamb to save a human. Of course its not moral.
I don't think it was moral because if he was God - which I don't believe he was - then he could have found some other way to save his friend. But no, the lamb had to die. If 'Jesus' loves all of 'his' creation, then why does he kill the lamb?