The student who declared his own mastery of the subject, his rejection and disdain for his teacher, or his refusal to learn was blind. No one can reject the Master and still call him or herself a Christian. And, the one who refused to do deepen knowledge and insight into faith faltered in their faith journey. When splinters of wood or sawdust flies, some will find their way into the eyes. If one carpenter asks another for help, its only natural to help the person in pain.
Negative criticism is neither constructive or requested.
In addition, the critic many times chides others to cover over a personal fault. Yet that judgement blinds us to our own faults, some of which can be truly major in scope. Jesus used an analogy from his own background as a carpenter. He pointed to the temptation in leaders to make negative, and sometimes rash, moral judgements. It was easier to point and condemn, than to take time to reflect, take stock, and reform.