If they are a bad Christian then 1. what does that mean??
3. If they admit they are not a good Christian and they stumble and fall then they are humble enough to recognize their weaknesses.
I think we're using the term 'bad Christian' for someone who claims to be a Christian, goes to church, does all the Christian things etc, but is otherwise a jerk - judgemental, not loving of others, impatient, etc, etc...
That's how I interpret that anyway.
As for admitting they stumble and all that - there's a difference between trying and failing, and not trying and calling it stumbling. A lot of people I've met in the church make no attempt to live a godly life and when someone calls them out on it, they pull the "I'm not perfect / Christians stumble / etc" excuse.
Chopsui can correct me if I'm wrong, but that's how I interpretted the question.