If Mr. Utterson is meant to be a virtuous friend to be a model so that Jekyll may become virtuous, how did he go wrong in the first place?

Utterson is not all together a sympathetic character. Utterson has few friends. Utterson does not really look out for friends or his fellow man. He is inclined to watch them travel down the wrong path without intervening.

"I incline to Cain's heresy. . . . I let my brother go to the devil in his own way."

It is probable that he cared little for Jekyll in the first place.