[Anime Spoilers] Everything I learned from this episode
The titan that was in Ilse's notebook is this dude
http://i.imgur.com/nrD9RHv.png
Titans are all humans that were unintentionally turned into titans and the only way to turn into a human again is by eating a titan. This is why Ymir ate Reiner and Bertolt's friend and also why the Ilse's notebook titan ate that one girl.
He thought it was Ymir so out of respect for her defending them and her being their "queen" he didn't eat her, when she started yelling at him and asking questions he knew it wasn't Ymir, that is why he was so sad and going crazy that he had to eat her in order to see if she is a shifter or not so he can turn back into a human.
The reason titans eat humans IS so that they can mindlessly test each human to see if they are shifters so they can turn human again. Someone/ Some group inside the walls as of right now know about this (im guessing the priests).
I am saying that because of this image
This shows an officer INSIDE the wall with a syringe about to turn Ymir into a titan to push her outside the wall so she can be a titan for for eternity as her punishment, along with all the other worshippers of Ymir.
Posted Jun 3, 2017, 4:38 PM UTC · edited