"At length" means after some time has passed, eventually. The speaker wants revenge. He has decided this with certainty. But the determination of his decision means he cannot allow any risk in getting his revenge. When he gets his revenge by punishing the man who offended him, he must do so in a way that protects himself from any further retaliation, punishment or harm. If person A hurts person B to punish him for doing something wrong, and person B can hurt or punish person A afterward, then the wrong thing that B did first has not been corrected/fixed/punished. If person A punishes person B for doing something wrong and person B does not notice that he has been punished, then the revenge is also a failure.
We learn by this that B has offended A, and A wants revenge, but he may be willing to wait. A not only wants revenge, but he wants his revenge to be done in a way that protects himself from harm and doesn't give B the opportunity to retaliate (or get further revenge on A). A also wants his revenge to be something strong enough that B will be really hurt and understand that A is punishing him for what he did.
Student or Learner