This will actually feel like a 2 part movie as they try and make two movies into one. I am thinking the reason for this was two-fold. One to tie in this movie with the rest of the movies from DC Animations. In fact if you watched Batman: Bad Blood, the end of that movie can be pasted into this movie. The other reason they likely made it a two part movie was to provide a filler to justify the time for this to be in theaters.
If you have seen the other Batman (New 52 based) animated movies from the past few years then you will kind of see where they were going but I can see why people would have been put off by this move by WB. Given how sacred people regard The Killing Joke, I think WB would have been better off just making it 2 separate segments. Basically give the fans who watched the movie in theaters a bonus film as a appetizer and finish with the Killing Joke.
At the end of the day I understood the tie in may have been to try and give more attention to a character in the piece but in doing so felt forced and a little out of place. The first part I would give it an average rating. But when you get to the Killing Joke, fans of the comic book is getting what they wanted. The pages of the comic on the screen voiced by Mark Hamill and Kevin Conroy. But the film may get tainted by the poor reception of the first part of the movie.
Overall this film is a rental. Smart move by WB to build up enough hype to get people wanting to check it out in theaters. But the beans have been spilled from the rumblings from Hall H. This is a nice film for comic book fans but will be better appreciated if they paid $4.99 (cost of a digital rental) vs $15 (a cost of a movie ticket).