When did the Eye of Providence start becoming associated with evil ?

If there is a one reason for this, I would pinpoint the cause to the seminal conspiracy satire novel 'Illuminatus!' by Robert Shea and Robert Anton Wilson, first published in 1975.

The namesake of the novel and the entity that now can be alluded to in many more or less serious conspiracy theories are 'Bavarian Illuminati', a secret society founded in May 1776 by Adam Weishaupt, a young professor of canon law at the University of Ingolstadt, in Bavaria. Today, an eye inside a triangle is almost invariably associated with the mysterious 'Illuminati' or some organization allegedly tied to them, with said Illuminati being concidered a global, secret pressure group capable of influencing major events, from market fluctuations to world wars. In reality, the 'Bavarian Illuminati' were a quickly developing group of Enlightenment thinkers, highly critical to the ideas of 'ancien regime' but it was nowhere close to influential, mostly on account of the internal strife between high-ranking members and infiltration by the state agents that lead to the banning of Illuminati themselves in 1785, after mere 9 years of operation. In addition, in the first half of their existence, the Order of the Illuminati was a small organization with no more than few dozen of members and managed to expand only after it was joined by Adolph Knigge, a noble courtier and former member of several masonic lodges belonging to the Rite of Strict Observance. With the experience and connections of Knigge, Illuminati quickly gained prominence and rose in size to over 1200 members, what meant almost a third of all members of secret societies in the Holy Roman Empire at that time. But this sudden popularity resulted in both internal strife with Knigge clashing with Weishaupt who allegedly was not eager to recognize contribution of the former and brought the attention of authorities that first banned an operation of all secret societies not chartered by the state in an edict of 22nd June 1784 and later specifically forbidding the operation of Illuminati and Freemasons in an edict of 2nd March 1785. At this moment it would worth noting that although they were often conflated, the Illuminati, Freemasons and Rosicrucians were generally considered as three different types of societies.

Secret societies were pretty much a standard fixture of the 18th-century urban culture, and although commonly criticized by the more conservative people, especially those who were not kindly disposed to the new ideas of republican government, they were usually not demonized. It changed significantly in 1789, when the revolution in France successfully managed to overthrow the absolutist monarchy and introduce republican government in a seemingly permanent manner. People with a substantial knowledge about the secret societies and hostile to their operation quickly either blamed them for the events on France, like priest Jacques François Lefranc in his book Le voile levé pour les curieux ou les secrets de la Révolution révéles a l’aide de la franc-Maçonnerie (The Lifted Veil or the secrets of the Revolution disclosed by the Freemasons) later popularized by a former French Jesuit, Augustin Barruel and John Robison, professor at the University of Edinburgh. The idea was quickly picked up not only by supporters of the ancien regime but also by e.g. Federalist Party in the newly formed USA that portrayed Republican-Democratic party as 'Illuminati radicals' who want to abolish Adams' government and possibly the Christianity as such.

It is worth noting however, that in the political polemics in late 18th and early 19th century, 'Illuminati' was presented as a secret political force, but their goals and methods were portrayed as destructive rather than constructive. In other words, they were depicted as conspirators striving to abolish the existing political or religious order rather than reach for power or accumulate influence. In a way, their depiction in conspiracy theories was not dissimilar to what will be attributed to 'anarchists' in the end of 19th century. The image of 'Illuminati' attempting to seize the power and control over the governments for some clandestine, selfish purposes emerged only later, quite possibly through the conflation with with the anti-Semitic conspiracy theories focusing on the alleged global influence of the Jews that came to prominence in mid-19th century and are best exemplified by the Protocols of the Elders of Zion that are largely copied verbatim from the satirical treatise on the policies of Napoleon III, titled Discussion between Machiavelli and Montesqieu in Hell and written by Maurice Joly in 1864.

With the emergence and then proliferation of the republican, democratic governments in the independent nation-states and popularization of the rational and even materialistic philosophy, the 'Illuminati' went into obscurity, as it became clear than what was considered a threat in 18th century was a political mainstream 200 years later and the 'backstage machinations' became largely associated with general Freemasonry. The conspiracy theories centered on the 'shadow global government' that attempts to bring 'New World Order' quickly proliferated after the Great War, in the wake of the great political upheaval in Europe brought by the October Revolution and collapse of absolutist empires, quick development of global capitalism, decolonization and emergence of United States as a global power. The global conspiracies were also quite popular among the proponents of new political models, especially in reference to Jews, often associated with global capitalism. These notions were further reinforced by the gradual emergence of the global political and financial organizations and the complexity of the worldwide economics.

Enter 1975, when Robert Shea and Robert Anton Wilson publish their first volume of the Illuminatus! trilogy, aptly named The Eye in the Pyramid that satirized various conspiracy theories that gained a lot traction in America during the 1950s and 1960s, and introduced 'Bavarian Illuminati' as a global conspiracy responsible for spreading misinformation, paranoia and complacency, an idea created by Wilson during discussion with Kerry Thornley, one of the creators of Discordianism, a satirical religion. According to Wilson, the eponymous image (please note that it is specifically mentions pyramid, not a triangle) is a direct reference to the reverse of a one-dollar bill (image of The Great Seal of the United States) and results from a conspiracy theory Wilson read in a letter to editors of 'Playboy' where he worked in the 1960s. Said theory was based on an assumption that Adam Weishaupt assassinated George Washington and impersonated him, using the USA to secretly spread Illuminati ideas. The fact that the Order of the Illuminati has been founded in the year USA declared independence and was disbanded few years before the French Revolution only helped.

The Illuminatus! theme was later picked by Steve Jackson, and American game publisher (not to be confused with a British game designer of the same name) who adopted an eye in a 13-level stone pyramid as his company logo) and created several games based on the mythos developed Shea and Wilson. The first one was Illuminati published in 1981, followed by several expansions throughout 1980s, a GURPS tabletop RPG supplement GURPS: Illuminati published in 1992 and a collectible card game Illuminati: New World Order published in 1994. These products gained substantial popularity and are often said to popularize Shea's and Wilson's novel, as well as the 'global secret society' themes in the mainstream fiction. In the first decade of 21st century, the symbol of the 'eye in a triangle' was firmly associated with global conspiracies and the name 'Illuminati'.

The Eye of Providence, as a symbol of the Divine presence that did not evoke a traditional, anthropomorphic presentation of the God, typical for the Renaissance art has been also adopted by the burgeoning Freemasons, although it was nowhere close to ubiquitous. It is true that the eye surrounded by sun rays and especially the triangle itself were common symbols used in Freemasonry, but the combination of the two is not that popular. The masonic diploma issued to Helena Blavatsky in 1877 contains these two items but they are separate. Lodge emblems and symbols adorning masonic documents from 19th and 20th centuries, one can easily find a compass and square (often with letter 'G' inside), sun and moon, two columns and sometimes skull and bones. Eye with sunrays is also quite common, especially in later specimens.