Sing Kwun was an infamous murderer and coward long before he ever became a Shaolin monk. He had murdered his student Golden Haired Lion King Tse Tsun's entire family in order to initiate feuds between the Ming Cult and the rest of wulin (all to avenge his lover having married former Ming Cult Leader Yeung Ding Teen). There's no doubt that he was evil.
So why did the Shaolin monks accept him as one of their own?
I can kind of understand that as Buddhist monks, they would try to practice the principles of mercy, forgiveness, and redemption (at least Hung Geen, who took in Sing Kwun, would...maybe not the others), but even if that were the case and they decided to provide Sing Kwun safe harbor at Shaolin, he should be a lowly janitor monk (not to be confused with THE Janitor Monk of DGSD fame). Instead, they promoted him to Yeun-class (which, under the Shaolin hierarchy of the time, was just one class below the reigning Hung class if you don't count the retired Dao class of elders). Sing Kwun had real influence at Shaolin. Chief Abbot Hung Wan always seemed to listen to his counsel. Sing Kwun should never have been allowed such a position of influence at Shaolin. It's how he was able to pull off his many manipulations and schemes during HSDS.
What was the Shaolin leadership thinking?