Why did Paul stand back and and give approval when Stephen was stoned to death in Acts 8:1? He approved of Stephen's death, but didn't join in with the stoning, he just guarded their coats (7:58).
Is it because he lacked the courage to throw stones himself? Was Saul essentially a coward who let others do the dirty work? Or is it that, as a Pharisee, he was determined to always do what is right by the letter of the law, and he recognised that the stoning of Stephen breached legal procedure.
Saul's active opposition is described in Acts 9:1-2. His hatred for the followers of Christ was murderous, but he got the paperwork right, and his intention was to "take them as prisoners to Jerusalem." (9:2)
Saul persecuted the church, but always did it by the book. He was a Pharisee, building his life on a legalistic righteousness. His mistake at the stoning of Stephen was that he failed to see his approval of murder as sin. He did nothing to stop the stoning, and didn't think that his inaction made him as guilty as those who acted to kill Stephen.
The things we don't do but should are no less serious that the sins we actively commit. Its Pharisaical to ignore the right we should be doing and call it obedience.