I really don't see why Ragnar couldn't have just declined Horik's proposition to raid without Jarl Borg, because it was breaking a sacred pact. Horik needed Ragnar for the raids (it would have been impossible without him), and if Ragnar wanted, he could have used it as leverage to keep their alliance intact. But, he was acting as Horik's lackey for the longest time, and I don't think Borg saw a difference between the two - which is most likely the reason, for why he attacked Kattlegat, in the first place.
The part that really got me though, was when Ragnar invited Jarl Borg into his house as a sign of "good will", but it ended up being a ruse to kill him. He didn't honorably challenge Borg to a duel, or meet him in battle. Instead, he chose to butcher him in the most cowardly and humiliating way possible.
Yeah, Ragnar's family suffered when they were being hunted by Borg, but none of them died. He reclaimed Kattlegat, and regained everything he lost, but he still wanted to appease his lust for blood. And it didn't just end there. Ragnar's men butcher all of Horik's children, except for his only son. He clearly didn't have any empathy when it came to other people's children.
Ragner has some redeeming qualities, but he strikes me as someone who craves fame and power, and won't hesitate to kill anyone whom he sees as a threat. He also doesn't like to share in the glory of their exploits - Floki designs his ships, and Rollo's the most powerful berserker in their army, but they're both conveniently left in the background.
As for the historical accounts, I think all of that pillaging, raping, and killing of the innocent needed some necessary repercussions
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