If there is one image that has come to epitomise Emmanuel Macron’s failed promise to restore the dignity and grandeur of the French presidency, it is the following: on the night of his victory, France’s newly elected president – the youngest French leader since Napoleon – took a solitary walk near the Louvres pyramid before walking up to the stage to the sound of the European anthem Ode to Joy. Observing the scene and following the president closely was Alexandre Benalla, Macron’s security guard, who would soon become a senior aide.

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Benalla is now under investigation for allegedly assaulting a demonstrator at a Labour Day rally in Paris on 1 May. In a video published by Le Monde last week, Benalla, who posed as a member of the French riot police, is seen on tape wearing a police helmet and hitting a man on the Place de la Contrescarpe, before leaving the scene.

Since then, other allegations have emerged: that Benalla manhandled a journalist from the Public Sénat network, and dragged a photographer out of one of Macron’s rallies during the presidential campaign. What has been labelled the l’affaire Benalla is now threatening to become the most serious political crisis of Macron’s presidency.

Play Video 1:12 Macron's security officer filmed beating protester – video

That the actions of an obscure security guard became une affaire d’état may seem surprising. Macron’s presidency has been riddled with political faux pas, especially in recent weeks – as the controversies over spending at his residences and remarks on welfare benefits have shown. This crisis is indeed different. It has exposed the double standards in Macron’s rhetoric, and the president’s willingness to protect his inner circle.

Benalla is accused of using physical violence against civilians and paraded in police clothes, two offences punishable in the penal code. Instead of reporting Benalla’s actions to the police authorities when compromising footage emerged, the president’s Élysée Palace suspended him for a few days (between 9 and 14 May), before bringing him back into Macron’s service.

By doing so, the Élysée administered its own sanctions and protected Benalla from judicial oversight. “We have the feeling that in Macron’s entourage, one is above the law,” Laurent Wauquiez, the leader of the French conservative party Les Républicains, quipped after the scandal flared up. It took Macron 36 hours to cave in to public pressure and announce Benalla’s dismissal on Friday.

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The affair has exposed the contrast between Macron’s calls for dignity and the reality of how politics is conducted in his entourage. It has made clear what several episodes in this presidential term had already allowed us to suspect: that Macron’s call for spotless behaviour comes with exceptions. Before the Benalla affair emerged, there were already signs of political trouble. Only a few weeks ago, anti-corruption police opened a preliminary inquiry into whether Macron’s chief of staff, Alexis Kohler, had violated conflict of interest rules.

The man who made “exemplarity” (morality beyond just following the law) a key theme of his presidential campaign, and promised to clamp down on politicians’ expenses is the same man who scolded a teenager in front of TV cameras for calling him “Manu”, referred to demonstrators against his labour law reforms as “slackers”, and is now accused of attempting to cover up his aide’s thuggish bevaviour.

Macron’s handling of public opinion has also betrayed his unwavering confidence in his ability to brush away scandal. Questioned about his aide’s conduct, the president dismissed the significance of the crisis, claiming that “the republic is steadfast”.

Play Video 0:34 Emmanuel Macron scolds teenager for calling him 'Manu' - video

Macron may believe that the actions of his aide do not affect him, but the political backlash he is facing from all sides of the political spectrum proves otherwise.

If the president’s closest aide parades as a policeman and beats up a man already lying on the ground, the word “exemplarity” rings hollow. The Benalla affair comes as the French police face increased scrutiny after a man was shot by the police in Nantes, sparking several nights of riots. Under public pressure, Macron decided to dismiss Benalla, but the controversy is unlikely to go away. The president’s belated response has compromised him.

• Cécile Guerin is a French journalist based in London