I agree that "redneck" is an insult, but it's not an insult based in race, color, national origin, or anything else over which one has no control. It's an insult based on attitudes, values, and lifestyle. It's a lot easier to say "redneck" than to say "lowlife, bigoted, beer-guzzling, misogynistic, ignorant, undereducated, gun-worshiping, trailer trash asshole".

As to why anyone would voluntarily choose to apply the term to themselves, please see "ignorant" and "undereducated" above.