When he was alive, McCoy "the Dixie Flatline" Pauly was a legendary console cowboy, or hacker. He received his nickname because his system hacks were so difficult he flatlined (read: died) during a couple of them. He was one of two legendary hackers who taught Case the ways of the console cowboy. If Case were Luke Skywalker, Dixie would be his Obi-Wan Kenobi.
By the time the events of the novel go down, Dixie has been dead awhile, but his memories live on in a ROM construct stolen by Molly and Case. ROM stands for read-only memory, meaning Dixie can't create new memories or learn or grow in any meaningful way. The ROM construct is simply the memories and instincts of a dead man that create a feedback loop, so he "always does what [you] expect him to" (17.38). Dixie finds his inability to change a total drag and asks Case to delete him once the job is done.
Case and McCoy have a few conversations on the nature of what it means to be a ROM (see 10.104-17 for an example), but he never explains why he hates being a ROM construct so much. We're thinking it has something to do with the big honking theme of change in the novel. These characters crave it, need it, want it, gotta have it. In a world that's constantly in flux, it must be kind of a drag to be the same old same old day in and day out. For Dixie, death is the better choice. After all, no one ever said death was a bore.