I understand how you feel but...
serial killers rarely confess (that would mean giving up control and feeling remorse which most of them are incapable of). Most are psychopaths, they don't think and feel the way the rest of us do. I don't think DM is capable of caring for anyone but himself (and he seems to enjoy hurting others - look at his behaviour with Tim and Laura's families in the courtroom.)
The only exceptions of serial killers confessing after being charged before conviction that I can think of are Russell Williams (who confessed for his wife's sake, he didn't care his vistims or their families) and Elizabeth Wettlauffer (the nurse) who seem to have had a small amount of guilt but only for herself spiritually. She also justified the murders to herself (the men were abusive, the women wanted to die and she was angry at all of them.)
If they do acknowledge their crimes after they are convicted, they usually don't take responsibly and blame something else for it. (ie Ted Bundy - *advertiser censored*).
They don't usually have a conscience or guilt about what they have done and don't care about it's effect on others (the victim's families or waste of court resources).