Secrets can be deadly and keeping one for far too long can make one do crazy things that in most cases will make one regret it in the end.
His greatest crime was that he was spineless and a great coward. Instead of protecting the one person that he loved, he ruthlessly watched hidden in a corner as his beloved, his secret crush got handled.
Like a coward, he could only secretly cry while in hiding and buried the love that never blossomed in his heart.
He still remembered the first time that he saw her, her smile was so bright that he completely fell at first sight. Her aptitude was very low and bad but she was very hard working and everyone saw that.
Maybe that was the reason why the elders didn't kick her out but let her serve in the sect even when she failed to break through the threshold.