Manwë and Morgoth were brothers from their very creation. Morgoth whom created evil and wrote into the Music of the Ainur, chords of discord that would affect the events of Arda to come, had corrupted many Spirits into his service and would lead them to war against the Valar in the early years and later against the Children of Ilúvatar in the lands of Beleriand in the First Age.
Manwë on the other hand was blissful and the wisest of the Ainur who came to dwell within Eä. He had no understanding of evil and therefore after the ages that Morgoth was imprisoned showed mercy to his brother even though Morgoth's malice was stronger than ever. How could Manwë not have foreseen the deception that his brother would soon play upon him or the Noldor Elves for that matter? How could Manwë have fallen for Morgoth's deceit in his hearing for his release when it should have been clear he was playing him and the other Valar for fools save Oromë, Tulkas and Ulmo?
Why did Manwë not understand the concept of evil and wasn't he aware of such deception within his realm of Valinor?