Well, I just finished monster today and I really enjoyed it. I thought I grasped the meaning to Johan suicide but after searching on the internet people were writing different interpretations so I would like you to hear my interpretation and correct me if I am wrong.
The second to last episode (episode 73) Johan and Nina recollected a memory in which Franz said that the twins were not monsters and are special. By recollecting this memory Johan attempted a suicide which will be led by Tenma's hand.
My interpretation into why Johan wanted to commit suicide is because of the recollected memory of Franz saying that they are special hence Johan at the moment of recollecting that memory he cried because that's when he realized what he was doing was wrong hence the suicide attempt and the reason why Nina forgave him. Am I correct?