Burley Keller roams around the country-side with the singular purpose of provoking gunfights.  His neglected and mistreated wife understands that the only thing he cares about is the look in a man's eye when he guns him down.  When he comes to Laramie, he meets a proud young man whose wife has just had their first child and goads him into a gunfight.  When he's acquited at trial because he prodded the young man into drawing first, he tells Dan he wants to settle down in Laramie and give up his guns.  Although we wouldn't call Dan gullible here, he certainly kept more of an open mind about this obviously despicable psycho than was characteristic for him.  Burley's real aim was to call out the Marshal himself and kill him.  He has also made his interest in Lily clear.  Of course, Dan has to kill him in the end.