There are multiple theories as to why there was a big fight. One is that Seth and Horus were both up in the sky, Horus ruled by day, Seth at night. They were both on good terms and chillin' in the sky until Seth became tired of his power. So he and Horus battled, Seth being considered the evil side (because of the darkness of night) and Horus the "good guy". Another theory is that Seth killed Horus' father for the big chair (a.k.a. throne) and this made Horus furious. So they battled for 70 years, but eventually Horus came out victorious and sent Seth to the underworld. Aren't I a great story teller? Tip your waitresses I will be here all week.