Negan's arrival was predicated all the way back in Season 5 and as the final episodes ticked down to his arrival, people were rightly excited.
Robert Kirkman and Scott Gimple gave the explanation that the reason the cliffhanger was there was because whoever Negan killed is the start of a new story. The current story had just finished and so they found it a good ending point.
Also Kirkman loves cliffhangers, but that's besides the point.
I agree with them on their reasoning and disagree with the people crying outrage. I admit the cliffhanger had to grow on me before I accepted it. It was such a good cliffhanger and, therefore, extremely painful.
The story that was told in the finale was one of the breaking of Rick Grimes. Rick goes from being sure of himself to a shell of a man in the course of this episode. I will follow Rick's journey of revelation from Alexandria all the way to Negan.