Page 81 - BERESHIT

Kayin ascended the ladder of Divine service by building an altar and
bringing an offering. Yet, after seeing his brother’s offering accepted by
Hashem over his, he rose up and killed him. Why did Kayin act in such
a contradictory manner? Even more, why did Kayin answer back to
Hashem “Am I my brother’s keeper?” after he previously sought His favor
and offered a tribute?

It would seem that Kayin was inspired to come close to Hashem but did
not use the opportunity to further that connection and keep the fire
burning. His spark of inspiration was extinguished, and his bad character
traits pushed him to kill his brother and run away from Hashem.