the anticlimax was disappointing at first but after going through some of the panels saved in my gallery, it makes sense to me.

i had a feeling since the library scene that dr tenma did not have in him to kill someone, and we can see evidence of this throughout his journey to johan but the library scene solidified this for me. at the core he will always be a doctor who was meant to save life because to him everyone is born equal. this belief of his contradicts johan's, who believes everyone is equal at death. and by dr tenma killing johan, he would have approved(?) johan's belief and let go of his own. 

i do love a story which runs in a cycle, which shows the irony of life. so monster going back to the circle of dr tenma operating on johan and johan leaving the hospital (since the ending is open, i interpret it as johan leaving the hospital upon dr tenma leaving the hospital premises as the story goes in circle we are back to square one) shows the cruelty of fate which is not in the hands of the holder. this is also shown by johan getting killed by a drunk man and not by dr tenma. he wanted to die at the hands of dr tenma but since he can't control fate, he was shot by someone else. 

overall naoki urasawa created a compelling storyline which has me wanting to jump in the naoki urasawa universe rabbit hole and think of the story in indefinite ways.