The scribes and Pharisees were diligent to make proselytes – those who came over to their position, disciples. These disciples were not Jews themselves, but were brought into the Jewish religion from outside.
Hypocrisy is indeed a dangerous sin. It can hijack even the best of endeavours. Even our zeal for God, our service to him, and our zeal to win others can be made to serve hypocrisy, and we must be so careful. If we draw attention to our service to God, or try to use it to hide something else, we may fall into this. Yes, we may need to set an example to the church in serving God, but how careful we need to be that we do it to please him and not to draw down the praise of men on ourselves.