James may have been snickering about it with Sirius all afternoon, suddenly realized that Remus could kill him and thus be put down and get Sirius expelled.

Yes, that seems far nearer the mark than 'James realises the sanctity of human life and that Snape's life, while being less worthy, since it's that of a creepy Slytherin; is still of a value. Perhaps the redemptive powers of Lily's love transformed him.'