I'm not really clued on them as a football enterprise. The only thing I know is that they run a multi-club system, which lots of owners do these days. What is about the Red Bull group that is so bad that Klopp is getting pelters for simply working there?
I'm going to give you a very German perspective, so focusing on Leipzig mostly:
RB created an artificial construct that took the place of established traditional clubs and only exists as a marketing vehicle for Red Bull. That's why a lot of fans dislike them. To look at their history, Rasenballsport Leipzig (a word that doesn't really exist but was created to allow them to shortcut as "RB Leipzig", as company names are not allowed as part of club names) actually took over the license from "SSV Markrahnstädt" - a completely irrelevant suburb club. There are well established clubs (Chemie Leipzig, Lokomotive Leipzig) which had a lot of trouble in recent years, bankruptcy etc. Still they are the traditional big teams and I think many people would have had no big problem if Red Bull had taken such a traditional club and helped them out of their misery.
Another reason for criticism is how they are exploiting kind of a loophole in the quite strict "50+1" rule in Germany, which states that the actual club (so, members of the clubs) have to own the majority of voting shares for the football operation. A typical BL club has tens of thousands of members, some much more (Bayern about 360,000 members as the actually biggest club in the world now, but Dortmund, Schalke, Frankfurt, Köln, Stuttgart, Hamburg and even Mönchengladbach have at least 100,000 members each). Some of these clubs sold a minority of their football operation to investors, but the rule is that it has to be a minority. So how did Red Bull get 100% control of RB Leipzig? By founding a club that essentially only excepts Red Bull management as full members. It's a brilliant move to effectively circumvent 50+1 while adhering to the letter to this rule, but it reinforces a lot the sentiment that RB Leipzig is not a real club.
Before you ask: Yes there are exceptions to the 50+1 rule. Currently only Wolfsburg and Leverkusen. Both exceptions were granted on the base of the long-standing association of the clubs to the companies Volkswagen and Bayer. Effectively the clubs were founded by the workers and such company sports groups have long existed and always had their place in German society. So they weren't founded for marketing in the same way as RB Leipzig was. Until recently Hoffenheim was another exception where Dietmar Hopp owned the team, but in this case in the end it was a billionaire who funded his own youth club from his little village until they reached the Bundesliga. If you want you can see kind of a romantic story here, despite him clearly being an a*****. By now he returned the shares to the club, but that was actually the only club in Germany that had a similar ownership model like it is usual in the PL. All three of these clubs are disliked and here it comes mostly down to the fact that they all were promoted far beyond their "natural potential" if we look at how many fans they have etc, so are taking the place of clubs that have more fans and do more for football culture.
I was gutted when Liverpool hired him, and it went better for them than I even feared it might.