What are the possible motives in The Devil in the White City? Why is understanding Holmes' motives important?

Two possible motives for the crimes committed by H. H. Holmes are profit and power, because he gained both by committing acts of fraud, torture, and murder. It's important to understand the motives of the monsters that walk among us, because not only does it help us avoid them, but understanding may also help us reign in our less than perfect impulses.

Erik Larson, the author of Devil in the White City, admits that "exactly what motivated Holmes may never be known." Holmes himself attributes it to birth. Larson writes:

"I was born with the devil in me," Holmes wrote. "I could not help the fact that I was a murderer, no more than the poet can help the inspiration to sing." Despite that, his actions and the results of those actions can help readers understand his motivations for undertaking such heinous acts.

H. H. Holmes is first exposed as a dangerous criminal when he locks his sister-in-law in an airtight vault that doubles as a gas chamber. When he purchased the hotel where he lives in Chicago, he hired several builders so that none of them would know his intentions—to build a house of horrors. 

Holmes murders people to collect their inheritance. He murders them to collect money he knows he can access after their deaths. He murders them for no reason at all. He married women for their money and murdered them when he aroused their suspicions. In short, he murdered for convenience and financial gain—but neither of those inspires a man to create a hotel with chutes leading to the basement, gas chambers, and torture rooms. That can only feed his taste for sadism and control.

Ultimately, the best reason to learn about monsters like Holmes—and to ascertain their motivations—is because it can help us spot monsters. If you know that a charming grin can conceal bad intentions, you might not be so quick to follow a man to his hotel or turn to get a newspaper from a vault. Understanding people like Holmes can also help us better understand our own motivations. While most people won't do things as awful as Holmes, profit, pleasure, and control can be powerful motivations. Knowing what terrible things these desires can drive may help us to only go after them in positive, healthy ways.

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.