It's been a while since I read this book, but I remembered reading something from the author regarding Holmes's motives. Here it is:
"Exactly what motivated Holmes may never be known" [p. 395].
We know he was content at times, but not always. We know he was a trained doctor who apparently enjoyed the "science" of death and dying and decomposition. We know he moved deliberately to be in a place and time where he could readily supply himself with fresh victims whose disappearance would probably not be noticed for a long time, if at all. We know he was ingenious at eluding capture for all manner of crimes, big and small. We know he didn't appear to have much remorse for what he was doing--not surprising for a man capable of such attrocities.
So, all that being true, we still don't really know what prompted him to walk this particular path in life. In the end, I guess that doesn't really matter very much. What would we do with the information? Even knowing, we still wouldn't completely understand his mental derangement. If we thought it might help us stop the next Holmes, we'd be wrong, as the essence of such a derangement is secrecy and deception--we still wouldn't know it until it was too late to do anything about it.
I guess I'm content to know the what and not the why.