The character of Roger Chillingworth is quite smart. He knows how to blend in, infiltrate through networking, and subtlety accomplish his goals. This being said, it would be quite easy to defend him in a trial.
First, there is no evidence in the novel that shows any attempt by Chillingworth to inflict any physical harm that could be shown as evidence in a court of law. He is clear, from the very start, that his revenge will not be attained in a concrete or physical way; his, is an entirely psychological and subtle attack whose effects simply cannot be proven beyond a reasonable doubt.
Second, there is more evidence of Chillngworth doing good than harm! In chapter IV, "The Interview", he actually uses his intelligence in herbs and the making of medicine to help both, Hester and Pearl! Chillingworth ascertains that he would never hurt Pearl nor Hester by making a poison for them, instead of a medicine.
What should ail me to harm this misbegotten and miserable babe? The medicine is potent for good; and were it my child,—yea, mine own, as well as thine!—I could do no better for it
In a trial, this could be entered to defy a motive; if he had wanted to kill both women right on that very day when he discovers Hester's shame, he could have easily done it by concocting a drink that would have killed them, either quickly or slowly.
Then, there is Dimmesdale. It would be simple to defend Chillingworth against any accusation of harm done against Dimmesdale because Dimmesdale had already shown signs of decay for a very long time, and prior to the arrival of Chillingworth into the village. Even though the appareance of Chillingworth causes Dimmesdale fear him, it is Dimmesdale who inflicts harm upon himself through self-mutilation. Moreover, Dimmesdale even says that his ailments are not to be cured through physical intervention; that something else is gnawing at him. This alone proofs that Chillingworth could not have done to Dimmesdale anything worse than what Dimmesdale had already done to himself. If anything Arthur Dimmesdale, and he alone, is his own worst enemy.
Another good defense for Chillingworth can be found in Chapter X. The narrator, in an omniscient point of view, describes how, regardless of his looks and his motives, Chillingworth has always lived his life in temperance and in a civilized way.
OLD ROGER CHILLINGWORTH, throughout life, had been calm in temperament, kindly, though not of warm affections, but ever, and in all his relations with the world, a pure and upright man.
During trials, it is customary to look back into the life of the accused to search for previous patterns of bad conduct. As chapter X shows, there are no such patterns found in Chillingworth's past. Hence, if he has done done harm in the past, nor in the present, who is to say that he is a bad man?
Therefore, the best defense would be to appeal to the reasons behind Chillingworth's anger; an anger which is actually quite understandable. The other best defense would be to present him as a man who has had many opportunities to cause harm and has not once done anything like that. Finally, show how that Hester and Dimmesdale's rash life choices may be a bigger danger to themselves than Chillingworth will ever be.