Continuity mistake: This is in the same universe as the other Arkham games yet Poison Ivy's gone from being green in Arkham Asylum and Arkham City to being just regular looking in this game.
Batman: I am vengeance. I am the night. I am Batman.
More trivia for Batman: Arkham Knight
Answer: He might have used his disguised dollotrons to place the bodies or simply disguised himself when sneaking out. As Gotham is in a criminal turmoil and most of the streets are dark and empty, it wouldn't be that hard to move around in the shady alleys unnoticed. And why did he do this? Simply because he's insane. An obsessed, maniac psycho-and sociopath, also schizophrenic beyond saving. Placing the bodies on display like that is part of his "artwork," presumably showing the world how imperfect those ones were in his eyes.
Dangar