Blimey, yes where to start on this. It's such a big thing that I think there's phd's written on the subject. This book has never been published in paperback but were it I'd probably give it a go though I can't imagine it's a light read.
Martin Torgoff's Latest Book - Bop Apocalypse: Jazz, Race, The Beats, and Drugs
www.martintorgoff.com
There's loads of people who met Davis who came away underwhelmed with him as a person and you have to assume the drug use contributed to that. Does that tarnish their legacy? Some bucked the trend, why did Gillespie manage to avoid the perils when so many around him didn't? Cab Calloway produced songs about drug use but apparently didn't take them himself.
In some ways the drug use mirrors the changes in the music itself. In the swing era in was marijuana but then when bebop appeared it mutated into heroin use. Loose music loose drugs, hard music hard drugs.
With Coltrane (who is my personal favourite) the sadness was that he did manage to kick the habit but it's highly likely by then the physical damage was sufficiently great that it was this that led to his early death.