Sitting around thinking why so many protagonists in crime fiction, at least at the darker ends of the spectrum, have protagonists who are addicted in some way. From Sherlock’s 7 per cent solution to the “dull routine of existence” to Nero Wolf’s beer to Philip Marlowe’s 
Old Forester, the idea that a crime novel’s protagonist must somehow be addicted has carried a lot of currency. My own protagonist, Elder Darrow, is an alcoholic in both Solo Act and In Solo Time, though practicing less and less as the books go on.
I suppose you can look at it, in character terms, as the necessary fatal flaw a protagonist must have for us to sympathize with him or her, something to make the protagonist human enough for us to want to travel on their journey. But why do so many of our main characters own an addiction? Is it because something like a drug or alcohol problem is common enough that everyone can sympathize? And if so, is a substance addiction the best way to make a character, as is said so clumsily, relatable?
I got thinking about this more when I read David Swinson’s book, The Second Girl, which features Frank Marr, a retired DC police detective with a serious longstanding cocaine habit. 
While the drug problem gives the character a certain edge, it’s such a pervasive part of the character that it made it hard for me to relate to him. I’m not trying to turn every character into a Sammy Sunshine but I’d like to feel as if the character at least recognizes his problem. Marr doesn’t seem to, just manages his way through the dope. Which leaves him dark, dark, dark. Not too much hope there.
So is an addiction required for us to relate to a protagonist and if so, does the addiction have to be cast as a weakness? Lucas Davenport, the protagonist of John Sanford’s Prey novels, is addicted to the hunt and to the violent end of his criminal prey. 
If an addiction is more intrinsic to character and less a characteristic like eye color or body odor, does that make the character stronger, more sympathetic to the reader?
If I had to guess, I’d opine that one of the things any addiction in a character provides is a way to keep a reader awake and aware at times when the plot isn’t rocketing ahead, in those inevitable places where the action must subside a bit to give everyone a breather. The addiction in a character says: “What bad decision are you going to make next because of this flaw?” And that decision may not necessarily be fueling the main plot.
I am beginning to think, though, that the subtler kinds of addiction or obsession—to one’s moral code, to one’s character flaws, to one’s deepest desires—make for more interesting protagonists than a single tangible characteristic that may not even be unique to the character. Or perhaps it’s that the addiction needs to be integrated into the protagonist in a way that makes him or her unique, or at least more interesting. Matt Scudder comes to mind—though the bad behavior caused by his addiction is mostly in the past, it has a way of affecting the way he operates in the books’ present.
Integrity of story is what it comes down to. Any addiction or obsession you provide a protagonist (or any other character, for that matter) has to be integral to the character, not a bolt-on like hair color or height. Or a plain yen for Scotch.
Kait Carson
Charlene D’Avanzo 
I think you can add to your list blindspots and fears–both rational and irrational (think Indiana Jones and snakes). Also the damage done by past losses. When I started writing the Thea Kozak series, I gave her a tragedy in her past that keeps her from moving forward. Burgess avoids relationships because he fears he carries the seeds of his father’s violence in him. But it is a very interesting question how far we go before we lose the reader to the “yuck” factor (as in your DC example) or we beat the reader over the head and turn then off.
Kate
Another thoughtful post on an important issue in crime fiction, Dick. Addiction as character flaw has been done, and done well, but is easy to do badly. I think you do it well in your Elder Darrow books precisely because Elder has self-awareness about the reasons for and consequences of his addiction, and he is trying to do something about it. That makes him an appealing character. Like Kate, I think there’s lots to be mined in the blindspot/fears area. Writing that into a character is more difficult, but ultimately more satisfying, both for writer and reader.
As a reader what I find attractive in a protagonist’s flaws (Does that sound as awful to you as it reads to me?) is that it makes them more human.
Unlike superheroes who have a unique superpower which solves all their problems, a flawed human protagonist is relatable.
Either we see ourselves in them or we say there but for the grace of God go I.
I also find it interesting that many protagonists’ partners or sidekicks lack a flaw except possibly being boring. No disrespect to Dr. Watson, but when was the last time you saw his adventures on a bookshelf anyplace?
Kate makes a good point about how far should a writer go with a flawed character. I have no data to substantiate this but most main characters, no matter how flawed they may be, stop well short of being anti-heroes.
Humans with all their flaws make for great stories especially when a couple of dead bodies are thrown in!
I’m not an author, but as a reader, I can say that I personally don’t find characters with addictions “relatable.” I realize that’s probably due to my own family history of having been raised by two alcoholic parents, and having other close family members whose lives have been destroyed by drug and alcohol addiction. For me, it’s very off-putting and I’m not likely to want to follow such protagonists. Just my own point of view….
I agree with seabluelee in not finding these characters “relatable”. Our son died of a heart attack, his heart problems probably caused by a cocaine addiction. We lived with the problems caused by addiction and I have no desire to re-live it through the books I read. I own a number of series books that I find very interesting without the need of this “flaw”.