Most of us hate Sega because we actually owned and LIKED playing the games on its consoles (most of SEGA's games rock), but then got screwed over because we bought the consoles just before;
(a) the company ceased support for that particular console in order to concentrate on whatever new project they would eventually fail, or
(b) the company bailed out of the console business just to cover its debts from Example A.

Obviously, somebody who dislikes games published by Sega, either with or without good reasoning.

Someone who hates Sega, either because they are ignorent, think they make crap games, or more commonly, hates sega because they fail to accept their own responsablility for buying a console/add-on which then failed.