Mike Yaklich adds to this background to this atrocity:
The Legionaires (strictly speaking, the "Iron Guard" were their military/"self defence" wing, although the term was often applied by contemporary journalists and later historians to describe the movement as a whole) did indeed revolt against Antonescu. The straight-laced, no-nonsense soldier Antonescu and the violence-prone radical right populist movement had co-existed in an uneasy ruling alliance since Antonescu had helped to depose the corrupt and incompetent King Carol in September 1940 (following the devastating cessation of Bessarabia and its large Romanian population to the USSR). The "Iron Guard" had elevated martyrdom to a virtual cult within their ranks. They had been largely founded by a charismatic leader named Codreanu, known to his followers as "The Captain" ("Capitanul"). The "Iron Guard" were highly anti-Semitic, nationalist, by their own lights deeply Christian (their processions on horseback through peasant Romania, wearing their green shirts marked with a cross and carrying candles, Codreanu up front on a white horse, made a deep impression of piety and purpose in many of the villages they passed through). Their simplicity of lifestyle and denunciation of the institutionalized corruption of the Romanian government gained them widespread respect and support among the common folk of the countryside in particular. The reaction of Carol's government to the growing popularity of the movement had been predictable. They had simply been barred from participation in the electoral process. The reaction of the "Iron Guard" was perhaps something Carol and his cronies had not predicted-- shortly after the first elections in which they were not allowed to participate, three "Iron Guard" gunman shot down and killed the newly elected Prime Minister. This began an escalating circle of violence and retribution between the government and the Iron Guard. After four years of it, Carol (using the highly-repected Antonescu as a go-between) negotiated a truce. Codreanu and his followers were welcomed back into the normal elective process in return for a "cease-fire." But it took less than two years for Carol to double-cross Codreanu, toss him in jail on trumped-up charges, and then, in 1937, to have him and a number of other imprisoned Iron Guardists murdered by their guards. Thus, when the Iron Guard finally got their shot at power in 1940, they had already lost most of their top (and arguably best) leadership,** and they were a very angry group of people. By this time the movement had become almost totally devoted to revenge and violence, as they showed already in a couple of disgusting outbreaks (the nastiest of which followed the discovery of Codreanu and the others' decomposed remains, under some rocks in the prison yard, where sulfuric acid had failed to completely dissolve the skeletons, in November 1940).
Antonescu, on the other hand, was a tough but (especially by the Romanian political standards of the day) honest and honorable military man, a hard-nosed conservative who was an advocate of law and order. He chafed at the repeated public brutalities of the Iron Guard (murdering random Jews in batches or specific former government officials in particularly slow and grisly fashion were their usual ways of proceeding) and their general lawlessness, and rebuffed attempts by the Guardist to expand their influence in the government. In January 1941 the situation came to a head, and the Iron Guard (which had become more a leaderless rabble than a movement) rose against Antonescu. The Army supported Antonescu, and, just as important, so did Hitler (overruling some SS types who were intriguing with the Iron Guard-- Hitler apparently valued peace in an important resource area over the spread of Nazi ideology to the region). The resulting events were short but bloody, as the Iron Guard launched another massacre and then were rather quickly scattered by Romanian troops.
Antonescu remained one of Hitler's more faithful allies, although he often disagreed with both his military strategies and his political programs (for instance, the Jews inside Old Romania were protected from deportation to German camps), ultimately leading to his own overthrow and subsequent execution.
Regards
Mike Yaklich
*Even as a colonel during the First World War (but as chief of operations for the Romanian general staff) Antonescu had masterminded some of the most successful Romanian operations (also in securing Transylvania shortly afterwards). His direct manner alienated many in the Romanian hierarchy, as did his penchant for unwavering honesty in political transactions, and, although he had also been defence Minister for a time Carol eventually had him jailed, fearing him as a potential threat to his rule. In this the King was prescient, for no sooner had he freed Antonescu from jail as a political sop to his critics than the latter helped overthrow him.
** Codreanu's character was a complex one, as a man who seemed the embodiment of upright moral principles and simple Christianity in action, but whose idea of combining fun with political action during his student days was to go and beat up Jews in the streets of his university town.