Parece que a decisão de Voldemort de ir ao Ministério estava relacionada à perda da profecia. Note que quando Harry está falando com Bellatrix no começo do Capítulo Trinta e Seis, ele, Harry, 'sente' a reação de Voldemort à profecia sendo destruída (minha ênfase):

“Potter, I am going to give you one chance!” shouted Bellatrix. “Give me the prophecy — roll it out toward me now — and I may spare your life!”

“Well, you’re going to have to kill me, because it’s gone!” Harry roared — and as he shouted it, pain seared across his forehead. His scar was on fire again, and he felt a surge of fury that was quite unconnected with his own rage. “And he knows!” said Harry with a mad laugh to match Bellatrix’s own. “Your dear old mate Voldemort knows it’s gone! He’s not going to be happy with you, is he?”

Momentos depois, Voldemort aparece:

“Don’t waste your breath!” yelled Harry, his eyes screwed up against the pain in his scar, now more terrible than ever. “He can’t hear you from here!”

“Can’t I, Potter?” said a high, cold voice.

Harry opened his eyes.

Tall, thin, and black-hooded, his terrible snakelike face white and gaunt, his scarlet, slit-pupiled eyes staring... Lord Voldemort had appeared in the middle of the hall, his wand pointing at Harry who stood frozen, quite unable to move.

Isso não parece ser uma coincidência. Aparentemente, ao saber da destruição da profecia, Voldemort mudou seu plano. Podemos, talvez, encontrar uma pista sobre o que é isso:

“Be quiet, Bella,” said Voldemort dangerously. “I shall deal with you in a moment. Do you think I have entered the Ministry of Magic to hear your sniveling apologies?”

Aqui Voldemort diz o que é não a razão pela qual ele veio. Isso nos diz que há algo mais que ele veio fazer. E a próxima coisa que ele faz é tentar matar Harry:

“I have nothing more to say to you, Potter,” he said quietly. “You have irked me too often, for too long. AVADA KEDAVRA!”

Então parece que o novo plano dele era simplesmente se livrar de Harry de uma vez por todas. Neste ponto, seu ardente desejo de erradicar Harry aparentemente superou sua cautela. Além disso, ele planejou fazer um ataque estilo hit-and-run, como fica evidente em sua conversa com Dumbledore:

“It was foolish to come here tonight, Tom,” said Dumbledore calmly. “The Aurors are on their way —” “By which time I shall be gone, and you dead!” spat Voldemort. He sent another Killing Curse at Dumbledore but missed, instead hitting the security guards desk, which burst into flame.

Assim, ele pode não estar muito preocupado em ser visto (e / ou preso) nos poucos momentos em que planejou estar lá. E ele também pode ter ficado com a impressão de que os Comensais da Morte asseguraram as instalações do Ministério, de modo que isso poderia ter tirado um pouco de sua cautela também. Além disso, ele não teria necessariamente outra oportunidade fácil de atacar Harry.

O Lorde das Trevas provavelmente saiu de sua raiva.

Quando o Lorde das Trevas aparece no Ministério, ele está furioso porque Harry quebrou a profecia e os Comensais da Morte não o impediram. Ele claramente não planejava ir, mas foi assim que percebeu que seu plano havia sido frustrado e que não podia mais obter a profecia.

“He can’t hear you from here!’

‘Can’t I, Potter?’ said a high, cold voice. Harry opened his eyes.

Tall, thin and black-hooded, his terrible snakelike face white and gaunt, his scarlet, slit-pupilled eyes staring … Lord Voldemort had appeared in the middle of the hall, his wand pointing at Harry who stood frozen, quite unable to move.

‘So, you smashed my prophecy?’ said Voldemort softly, staring at Harry with those pitiless red eyes. ‘No, Bella, he is not lying … I see the truth looking at me from within his worthless mind … months of preparation, months of effort … and my Death Eaters have let Harry Potter thwart me again …”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 36 (The Only One He Ever Feared)}}

Parece mais provável que ele tenha aparecido no Ministério de raiva e não tenha tido nenhuma razão lógica para fazê-lo - ele provavelmente queria punir os responsáveis, e sua fúria anulou seu sentido. A profecia já havia sido destruída e haveria lugares menos arriscados para tentar matar Harry. No entanto, ele provavelmente queria punir os Comensais da Morte e matar Harry por fazer com que seu plano falhasse, e sua fúria sobre a situação levou-o a ir impulsivamente ao Ministério. Ele também se irrita quando descobre que sua Horcrux foi tirada de Gringotes.

“It was … it was … the P–Potter b–boy and t–two accomplices …’

‘And they took?’ he said, his voice rising, a terrible fear gripping him. ‘Tell me! What did they take?’

‘A … a s – small golden c – cup m – my Lord …’

The scream of rage, of denial, left him as if it were a stranger’s: he was crazed, frenzied, it could not be true, it was impossible, nobody had ever known: how was it possible that the boy could have discovered his secret?

The Elder Wand slashed through the air and green light erupted through the room, the kneeling goblin rolled over, dead, the watching wizards scattered before him, terrified: Bellatrix and Lucius Malfoy threw others behind them in their race for the door, and again and again his wand fell, and those who were left were slain, all of them, for bringing him this news, for hearing about the golden cup –”
_{^{- Harry Potter and the Deathly Hallows, Chapter 27 (The Final Hiding Place)}}