The tricky part that confuses everyone is the memory of the Rose Mansion because it is unknown to whom it actually belonged to. Was it Anna or Johan?, and for the Mother's case, clearly she was under so much pressure that she could had committed the mistake.
Secondly the way the twins were dressed both as girls probably suggests their mom knew this was going to happen. So for her sake and to prevent any emotional attachment while taking decision and to fool her mind or herself, she made them dress that way but there is no denying that Johan had a mindset of a psychopathic killer that he had exhibited from the times of 511 Kinderheim, however for me the real plot says the world was the monster to the twins. I support this by saying that it was the experiments that made their mindset corrupt. It's not like the twins were born with a killer mindset were they?
Secondly, about the last episode it is clear the mother is herself, however she is not sure which twin she choose to let loose, nor is Johan or Anna. It is also clearly evident that they were both blaming any one of these three as "The Real Monsters" which is foolishness. It's The situations created by Nazis and Humans that were "The Real Monsters".
A third point to consider is the empty bed at hospital and it's possible that Johan committed suicide. After all his whole plan was a "Perfect Suicide" or he could have possibly escaped and remained underground all throughout his life because he finally got a name, so he was no longer "A Nameless Monster" anymore and that is what I think happened at the end.