Being in prison ministry, I see first hand the effect that just "regular" prison has on marriages, and it's not good. Some marriages do survive the prison experience but they're few and far between. And generally it's the wives on the outside who try to preserve a marriage; rarely the reverse. Death Row, of course, magnifies that situation beyond belief.
While it's always possible that Darrin is an extremely dedicated loving husband, it's more likely IMO that Darlie's got something on him. I've never believed that Darrin was an accomplice to the murders; however, I do think he knows more than he's telling. And if what he knows could incriminate him, he'll be standin' by his woman till the bitter end