Kristi Noem once led a dog to a gravel pit and ended its life with the cold precision of a mafia hit. On Thursday, the homeland security secretary confronted the grim truth that she, too, was expendable.

Noem became the first cabinet member fired in Donald Trump’s second term, a striking contrast to the revolving-door chaos of his first. Like other members of Team Trump, she had assumed that ostentatious displays of fealty to the president would insulate her.

As the face of immigration enforcement, often putting herself in the centre of the action, she managed to turn Trump’s signature issue into a political liability. Add a disastrous appearance before Congress this week and it was enough for the president to finally resurrect his reality TV phrase: “You’re fired!”

Trump told a story on Wednesday about another cabinet member, the energy secretary Chris Wright, who recently made a speech in which, instead of merely mentioning “coal”, he used the phrase “clean, beautiful coal” every time in an effort to impress the president. “I said, Chris, easy!” Trump recalled to an amused audience.

Noem was to immigration enforcement what Wright was to coal. She tried too hard. There was performative cosplay: sometimes the 54-year-old dressed in a flak jacket and accompanied agents on immigration raids as cameras recorded, though she does not have a law enforcement background.

The former governor of South Dakota also posed in front of a group of shirtless, tattooed men behind bars during a visit to an El Salvador prison where the Trump administration sent people it accused of being gang members. On social media, she referred to immigrants convicted of crimes as “scumbags”.

Meanwhile, under her leadership, there was terror as masked immigration agents surged into Los Angeles, Chicago and Washington, scouring neighbourhoods and Home Depot parking lots in search of possible immigration offenders.

At first, Trump did not flinch, welcoming the hardline approach. But again, Noem went too far. The crackdown spiralled out of control in Minneapolis with sweeping raids and teargas in the streets. Trump understood how badly that played on his key metric, television.

Noem hastily said two US citizens fatally shot by federal immigration agents in Minneapolis had committed “domestic terrorism”, an assertion undercut by video evidence. Trump removed Greg Bovino, the architect of the operation who reported directly to Noem, and replaced him with border czar Tom Homan, with whom Noem had a long power struggle.

Noem also faced mounting criticism over the way the Department of Homeland Security (DHS) spent billions of dollars allocated by Congress – including two Gulfstream G700 luxury jets for $172m during a government shutdown – and over the pace of emergency funding approved through the Federal Emergency Management Agency for disaster response.

Then there were her personal scandals, including long-running rumours of an extramarital affair with top aide Corey Lewandowski, who was Trump’s first campaign manager in 2016. He, too, has now been dismissed. A US coast guard pilot was allegedly dismissed after one of Noem’s personal blankets was left behind on a plane – only to be reinstated because no one else was available to fly the return leg.

The trouble came to a head this week when Noem faced a two-day grilling on Capitol Hill and came in for rare, but blistering criticism from Republicans. One particular point of scrutiny was a $220m ad campaign that included a scene of Noem on horseback at Mount Rushmore in her home state of South Dakota.

Noem told members of Congress that Trump had been aware of the campaign in advance, but Trump disputed that in an interview with Reuters. “I never knew anything about it,” he said.

That was the last straw. Trump was “said to be especially upset” about Noem’s deceitful response, Punchbowl News reported, and asked Republicans whether he should fire her. Soon after, he announced on social media that Noem would become special envoy for the Shield of the Americas while Markwayne Mullin, a Republican senator from Oklahoma, would take over at DHS.

Ousting Noem was the right decision for the wrong reason. Elizabeth Warren, a Democratic senator for Massachusetts, responded on Thursday: “So let me get this straight: Trump fired Noem after she lied about spending over $200 million of taxpayer dollars on ads promoting herself. But he wouldn’t lift a finger after two Americans were killed by her federal agents?”

Indeed, many will regard Noem as the worst homeland security secretary yet seen. She demonised immigrants and inflicted misery by targeting non-criminal, working immigrants and families. The number of deaths in immigration detention rose to a two-decade high during her tenure while staff in DHS oversight offices were slashed.

Miles Taylor, who was chief of staff at the DHS during Trump’s first term, posted on X: “Kristi Noem will be remembered for treating the American people like she treated her dogs.”