Board Thread:Lore Discussion/@comment-24810427-20140401014505/@comment-13446185-20140401024258

Because the Forsworn are ethnocentric barbarians who will kill anyone who is not a Reachmen (though they do often kill Reachmen). As well as the fact that they unjustly usurped the Reach then acted like victims when the Nords took it back. Other reasons why the Forsworn should be hated is because they were originally used by the Silverblood family as a means of hectoring civilians of the Reach out of their land holdings (Silverbloods are terrible people).

Lastly I will quote the wiki article Reachmen (Lore), to show why Tribal Reachmen are not good people (Normally I'm against quoting wiki pages but I can vouch for this as it is well referenced (and I wrote it))

"They are also known to practice taboo magics such as human sacrifice and the creation of Briarhearts, this is the reason they are known as the "Witchmen of High Rock". It is believed that a lot of their taboo magics were discovered through trade between the Reachmen of the Western Reach and the Orsimer tribes of the Wrothgarian Mountains.[1] The Reachmen are also described as being very cruel, often kidnapping the inhabitants of villages and enslaving them, and some clans were known to sacrifice children by cutting out their still beating heart. Firsthand accounts say that even the Reachmen's humor was normally dark and malicious.[11] During times of war, the Reachmen were known to pillage, massacre, and raze any settlements they came across and were normally known to use overwhelming forces and speed when attacking cities rather than siege tactics, as they lacked proper siege weaponry and naval forces to do so.[4]"