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Disclaimer: no offence is intended to any of the puzzling.SE users mentioned (or not mentioned) here! I was inspired by this question to use them as characters, with no regard for reality.

You are walking through the mansion of the mighty Lord D'alar'cop, Duke of Puzzling in the vast empire of Stackexchange. You see almost nobody apart from the occasional servant. As you pass outside through the great double doors at the entrance to the mansion, you overhear part of a heated conversation between two footmen, Warspyking and Michael.

"His Lordship will hear about this! You'll be out on your ear before the end of the week!"

"Oh, will he indeed? We'll see about that. If you tell him anything about me, your adventures with a certain maid might also come to his attention."

"He'll never hear a word from you again, I can promise you. You..."

As you walk further out along the sweeping drive towards the edge of the estate, the footmen's conversation fades out of your earshot. It is November, and bitterly cold even at noon. The parkland surrounding you is covered in deep snow, but the drive has been kept clear of snow and ice for his Lordship's carriage. The winter panorama is astonishingly beautiful, and you do not even notice the cold.

You reach the Folly, a small tower just off the main drive where his Lordship is normally to be found at this time of day, poring over his books. A gardener in snowshoes, Oblongamous, is adjusting one of the snow sculptures that are dotted about the grounds, and fails to notice you as you walk up to the Folly and enter silently.

When you open the single door to the Folly, you find his Lordship sprawled over his writing desk in a pool of blood, clearly quite dead. His body is stiff and ice-cold; he must have been dead for some time. You wonder why nobody has yet found the body and alerted the authorities. Looking around for clues, you see a bloodstained knife lying on the floor just behind the door. Also the electric light is switched off, the only light coming from the sun shining directly through the open doorway. There are no windows. The killer must have turned off the light as (s)he left.

You leave the Folly, moving northwards across virgin snow towards the mansion. But you have only got a few metres when you hear a piercing scream from behind you. Looking back, you see a maid, Avigrail, running out of the Folly in a terrible state. Oblongamous is still nearby; when he hears her story, he checks quickly inside the Folly and then they both go running to the mansion to raise the alarm.

The bloodstained knife is found to bear the fingerprints of Gilles, a well-known rival of his Lordship, and the blood is of his Lordship's blood group. At Gilles's trial, his defence lawyer makes the case that his Lordship's death wound is much wider and rounder than the knife blade, and the bloodstains on the knife show it could not have entered the body. He argues that the blood was only smeared onto the knife, and finds a cut on his Lordship's arm which could have been used to produce blood for this purpose. "Someone," he thunders in his final summing-up, "has been trying to frame my client!" His eloquence impresses the jury, and Gilles is acquitted.

Avigrail and Oblongamous both appear as witnesses in court, and say they saw nobody entering or leaving the Folly other than his Lordship.

The police have nothing to go on, but you can help!

Who killed his Lordship?

Bonus question: how was he killed?