Frankenstein- Ability to reason

"'Listen to me, Frankenstein. You accuse me of murder, and yet you would, with a satisfied conscience, destroy your own creature. Oh, praise the eternal justice of man! Yet I ask you not to spare me: listen to me; and then, if you can, and if you will, destroy the work of your hands." page 69

It was at this point in the book that I began to see how intelligent the creature has become. The creature reasons with Victor about conscience and mankind. This is vastly different from the creature that we had been introduced to when Victor first created it. The baby-like creature who yearns for its creator has now transformed into an intelligent being. This was an extreme surprise to me. Originally I figured that the creature would continually behave in this babyish manner and not really develop any sense of intelligence, especially due to his abandonment. I do not think that Frankenstein thought that the creature would become intelligent and be able to reason and understand either. Another point in the first half of the book that has bothered me was the fact that Frankenstein never even spent time with the creature, but automatically knows that the creature killed William. I do not understand how Frankenstein can logically assume this because he has no true knowledge of what the creature is like.