Nothing New Under the Sun, Here as to Rules That Focus on a Defendant's Being Motivated by Ill Will

"[V]ery agreeable to the theorist, but utterly unfit for the practical purposes of society ...."

From Thomas Starkie's 1813 libel law treatise, pp. 199-200 (some paragraph breaks added):

Were every malicious and oppressive act to be considered illegal, the law would be very agreeable to the theorist, but utterly unfit for the practical purposes of society, on account of the infinite perplexity and uncertainty which would occur in distinguishing between bad and good motives, where the act done might arise from either source.

The secret purpose of a man's heart is of too difficult ascertainment to be made the general criterion of legal right or wrong; and hence it is that the law, which must resort to more plain and certain evidence, on which it may found more precise prescriptions, may not unfrequently become the instrument of oppression.

For example: where a creditor, from motives of malice and revenge, and not with a view to his legal remedy, deprives his debtor of liberty; as a matter of conscience the act is criminal and unjustifiable; yet it must be sanctioned and tolerated by the law, in order to avoid consequences most pernicious and absurd; for were it otherwise, a man's security for his debt would depend on the temper and disposition of his mind, upon his motive for becoming a creditor in the first instance, and his reasons for enforcing his claim in the second; and, however pure his intention, he might be deterred from proceeding, by the apprehension of being involved in litigation, upon a suggestion of malice, by his adversary.

And the same kind of reasoning is applicable to the case of slander: it would frequently lead to too nice and critical an investigation, to inquire whether a person, who published words in the course of performing a public duty, was actuated by malice and ill-will, or by conscientious motives; and much inconvenience would arise, were persons to be deterred from the performance of public duties, or from a fair prosecution of their claims, by the apprehension of the liabilities and danger to which they exposed themselves.

Despite this, defamation law in the 1800s and 1900s (at least in America) often allowed a defense of truth only when the statement was found to have been said with "good motives" and for "justifiable ends." It wasn't until Garrison v. Louisiana (1964) when this view was conclusively rejected, at least to statements on matters of public concern:

Debate on public issues will not be uninhibited if the speaker must run the risk that it will be proved in court that he spoke out of hatred; even if he did speak out of hatred, utterances honestly believed contribute to the free interchange of ideas and the ascertainment of truth. Under a rule … permitting a finding of malice based on an intent merely to inflict harm, rather than an intent to inflict harm through falsehood, "it becomes a hazardous matter to speak out against a popular politician, with the result that the dishonest and incompetent will be shielded." Moreover, "[i]n the case of charges against a popular political figure … it may be almost impossible to show freedom from ill-will or selfish political motives."