To explain this I have to go back a bit: The first of the in the meantime 6 clubs of their nowadays 6 club conglomerate was Red bull Salzburg, which they got by taking over the formerly fan-owned club Austria Salzburg, with eliminating this clubs democratic club structure and changing their name to Red Bull, whilst deleting the old club and his history from club register, against the will and ignoring massive protests of this clubs fans.Their second acquisition was German club SSV Markranstädt, a then 5th tier club that went bust. To make sure their take-off didn't violate German club regulations too obviosly they created a dirty trick. Since DFB, the German football association, already had the 50+1 rule in place,they couldn't just take over this club as owner and scratch member's voting rights. Therefore they changed the statutes of said club in a way that it's still member-owned officially yet restricted the voting rights to 23 members who had to be approvedf by the Red Bull concern, undermining the sense of 50+1 rule specifically, and the fundamentally democratic spirit of German association law regarding clubs generally, in the process. Now add that REd Bull benefitted from loop holes in DFB regulations and therefore didn't need to start in the lowest tier of the pyramide, like other new founded clubs, but in 5th tier and plastering their logo over everything formerly associated with the old clubs. That should explain , why they're hated. Also it's debattable whether forming an empire of football clubs in different countries isn't questionable in itself.
Disclaimer: If you're not aware what the 50+1 rule is, look it up. This post is one of the longest i've ever wrote on this forum without going into its details already.