Well there's ample supporting evidence it's within Ulfric's character.
Also, I don't believe Ulfric is an evil psycho. I think he can be a deeply complex character who is capable of doing many things both great and horrible.
* Remember, the whole "Sparing Nords" deal doesn't prevent Ulfric from attacking Windhelm. In fact, the Bear of Markath accuses Ulfric of being of the mind that, "if you weren't for us, you're against us" which is EXACTLY what he says about that city and its attempts to remain neutral.
* The Forsworn aren't worshipers of Talos but the Daedric Princes. That may explain why Ulfric felt the need to put down the Rebellion so harshly.
* Wasn't the Jarl of Markath the guy who HIRED Ulfric in order to put down the Rebellion? Promising that he'd be allowed to worship Talos freely.
* While the book is propaganda (war crimes? IN SKYRIM?! Do they even exist?), the brutality of the put down doesn't exactly fail to match actual Medieval attacks.
I just think it adds an interesting dimension to Ulfric's character.
1) I think you mean Whiterun, not Windhelm. Ulfric attacked Whiterun because of its strategic position. He did not, however, slaughter innocent people inside of it. He deliberately spared the Jarl's life, as well as his Housecarl's, even though Galmar Stone-fist wanted to kill him. I never said that Ulfric is completely against killing, I said he is against doing it when it is necessary, and he only kills combattants. He was actually MERCIFUL to the combattants at Whiterun, and Solitude, and all of the other places we took. If Ulfric is willing to spare the leaders on the side of the Empire, why would he just slaughter innocent people? Again, nothing whatsoever supports this.
2) Ulfric put down the rebellion so he could worship Talos. The Forsworn also worship Dibella. There is no evidence to show that Ulfric knew or cared about who the Forsworn worshiped.
3) Yes, the Jarl of Markarth hired Ulfric to take care of the Forsworn. That does not mean that Ulfric condones what the Jarl does, and everything about Ulfric that I have witnessed tells me that he wouldn't support the slaughter of innocents. The Jarl of Markarth killed innocent people. This makes the Jarl of Markarth a horrible person, not Ulfric. Of course, there are plenty of reasons to think that Ulfric is a horrible person. This just isn't one of them.
4) Yeah? And? Yes, other people have slaughtered innocent people. The question is whether or not ULFRIC did. And there is no evidence to show that ULFRIC did this. What other people have done doesn't matter. One of the Counts of Cyrodil has an Argonian torture chamber in his castle, does that mean that every person on the Imperial side does? No. You look at the evidence and character for each individual person and decide based on that.
Believe me, I was inclined to believe that Ulfric did this at first, just like I thought he was a Thalmor agent. Then I put 400 hours into playing the game (yes, I know I have no life), really looked at the character of Ulfric and observed his actions and learned his history, and I've decided that based on that it is highly unlikely that this book is true. Maybe there are kernels of truth in the book, but if they are they are greatly exaggerated and in my opinion, the credibility of the author is questionable. The fact is, in the Elder Scrolls, EVERY author of EVERY book is biased. You can never read one book to get the real story, you have to read several and see several perspectives to figure out the real truth. And it is sometimes a pain in the butt, but it makes the story incredible. Ulfric is one of the most interesting characters in Elder Scrolls lore in my opinion. But the Bear of Markarth simply isn't factual.