No-one is questioning why Negan did it, nor the fact that he did it in the way he did. That's the script and story, that's what the writers feel needed to happen to build the character. Everyone gets that Negan is a sadist and trying to break Rick.
The issue some people seem to have is how it was portrayed and shown, the screenplay choices. Did it need to be so graphic and more importantly lingering to build the character and tell the story? Could those storytelling ends have been achieved via more subtle screenplay and perhaps actually been more powerful as a result?
Everyone will have different opinion on this. But for me it descended into cheap shock horror in a way that previous killings in the show mostly haven't (even Hershel) and, for me, lost a lot of storytelling power because of it.
I guess that's always the balance they're always trying to juggle, between the brutal violence and the storytelling. For every person who thought EP1 went too far with the brutality, there's someone else who thought EP2 was dull because of the lack of it. But the problem with going down the shock horror route is audience desensitisation and the need to keep going one better.