Even if Felt is Deep Throat, we will never know much about his motives. He can't tell us anymore (time and age distort everything). More importantly, he can't tell us much about his dealings with Woodward. We have only one version of interaction-- W&B's myth-making in ATPM.
A couple of days before Vanity Fair released it's story I quoted Joan Didion's warning that "writers are always selling somebody out."
Investigative reporters often wheedle, cajole, flatter and threaten their sources to pry lose useful information. Here's a reporter looking at Sy Hersh's methods:
In those years, much attention was focused on Hersh's personality and reporting techniques. One of his editors at the Washington bureau, Robert Phelps, recently recalled, with wry disbelief, the kinds of messages that Hersh would leave. "He would call people and he'd say 'I'm Seymour Hersh, I'm doing a story on this . . . If he doesn't call me, I will get his ass.' They'd call back."So it is possible that Felt dropped a few bites of information to Woodward out of spite and disappointment, and then found that he was hooked by the reporter. It is also possible that Felt (who was convicted of crimes for his actions at the FBI) was caught between revealing dirt on Nixon or seeing the dirt from the Bureau show up in the post.
Would Woodward or Bernstein play that game? Who knows? They certainly won't tell us, and their compromised sources won't tell us. An uncompromised sources (one who is not trying to swap information to protect his own secrets) or a resolute non-source would have no idea about the sinister side of the investigation.