I really don't like Dean right now. I mean, a part of me understands why he killed the girl but he just made that kid an orphan and he will likely keep this a secret from Sam which won't help matters since them keeping secrets from each other always leads to something disastrous down the line. I have a feeling that Sam is going to find out, maybe during his trial in next week's episode. I think Dean often use Sam and his issues as a buffer behind him and his issues, and Cas's betrayal seem to have dampered his trust in supernaturals which Sam partially belong to, fearing that eventually they'll let you down. Dean seriously needs therapy. Years and years of therapy.