The killer is

Oblongamous

Since

"Oblongamous, was adjusting one of the snow sculptures that are dotted about the grounds" and is the Gardner, it will be easier for him to get an icicle or a sharp object made with ice, since it was snowing and also there are ice sculptures outside. Most probably, he should have taken it from one of the ice sculptures and may be that's why he was fixing one of the sculpture from which he took the sharp part.

Also

Oblongamous was working outside the folly with a snowshoes and was outside. Still there is no foot prints means he would have cleared the foot marks with a broom or something. Otherwise, there would be his foot mark.

So my assumption is

Gilles came to kill D'alar'cop from his back while he was writing, but when Gilles was about to stab him, Cop noticed him and blocked the knife with his hand which made the wound in his hand and blood on knife. In the fear that he will get caught, Gilles thew the knife there and tried to escape from the mansion. While he was running, Oblongamous caught him. Seeing Oblongamous , Gilles got some idea. He gave Oblongamous a handful of gold coins and asked him to kill Cop for him. Oblongamous agreed and put the coins in his pocket. He broke one of the ice sculptures and took a sharp piece from it. He entered the Lord's room and switched the light off so that even if he get caught, the Lord (Cop) won't identify him. He sneaked right behind Cop and stabbed him on his stomach with the icicle.

The killer is

Oblongamous

Since

"Oblongamous, was adjusting one of the snow sculptures that are dotted about the grounds" and is the Gardner, it will be easier for him to get an icicle or a sharp object made with ice, since it was snowing and also there are ice sculptures outside. Most probably, he should have taken it from one of the ice sculptures and may be that's why he was fixing one of the sculpture from which he took the sharp part.

Also

Oblongamous was working outside the folly with a snowshoes and was outside. Still there is no foot prints means he would have cleared the foot marks with a broom or something. Otherwise, there would be his foot mark.

So my assumption is

May be Oblongamous hated both Gilles and Cop and so he decided to finish the nuisance of both in a single shot. He stolea knife from Gilles house in the night with Gilles finger print. Oblongamous used gloves so that there won't be his finger prints and wont damage Gilles prints. Next day when Cop was busy with some writings, Oblongamous thought "This is the right time. There is nobody near him." He broke one of the snow sculptures and took a sharp piece from it. He switched the light of so that Cop won't identify him even though the attempt failed. He reached the back of Cop and stabbed him on his stomach with the icicle. Since the wound was deep and Cop was week (since he is too old ;p) Cop died very soon. Then Oblongamous made a wound in Cop's hand with the knife so that there will be Cop's blood on Gilles knife and dropped the knife there, so that the police will find Gilles prints and will arrest him. So he will get rid of troubles from Gilles too. After leaving the room he took a broom and cleared all the foot prints he made and clearing the marks he reached near the broken sculpture (he broke it to get the icicle). Before somebody noticing he tried to fix it and place it back urgently. That is when "YOU" reached there and saw that. Since Oblongamous was in a hurry fixing it before somebody noticing it, he didn't saw "YOU". So Gilles or no one else entered the mansion and that's why the Guards didn't see anyone. It was Oblongamous itself who killed Cop, switched the lights off and cleared the foot prints.

Another assumption is

Gilles came to kill D'alar'cop from his back while he was writing, but when Gilles was about to stab him, Cop noticed him and blocked the knife with his hand which made the wound in his hand and blood on knife. In the fear that he will get caught, Gilles thew the knife there and tried to escape from the mansion. While he was running, Oblongamous caught him. Seeing Oblongamous , Gilles got some idea. He gave Oblongamous a handful of gold coins and asked him to kill Cop for him. Oblongamous agreed and put the coins in his pocket. He broke one of the ice sculptures and took a sharp piece from it. He entered the Lord's room and switched the light off so that even if he get caught, the Lord (Cop) won't identify him. He sneaked right behind Cop and stabbed him on his stomach with the icicle.

The killer is

Oblongamous

Since

"Oblongamous, was adjusting one of the snow sculptures that are dotted about the grounds" and is the Gardner, it will be easier for him to get an icicle or a sharp object made with ice. Most probably, he should have taken it from one of the ice sculptures and may be that's why he was fixing one of the sculpture from which he took the sharp part.

Also

Oblongamous was working outside the folly with a snowshoes and was outside. Still there is no foot prints means he would have cleared the foot marks with a broom or something. Otherwise, there would be his foot mark.

The killer is

Oblongamous

Since

"Oblongamous, was adjusting one of the snow sculptures that are dotted about the grounds" and is the Gardner, it will be easier for him to get an icicle or a sharp object made with ice, since it was snowing and also there are ice sculptures outside. Most probably, he should have taken it from one of the ice sculptures and may be that's why he was fixing one of the sculpture from which he took the sharp part.

Also

Oblongamous was working outside the folly with a snowshoes and was outside. Still there is no foot prints means he would have cleared the foot marks with a broom or something. Otherwise, there would be his foot mark.

So my assumption is

Gilles came to kill D'alar'cop from his back while he was writing, but when Gilles was about to stab him, Cop noticed him and blocked the knife with his hand which made the wound in his hand and blood on knife. In the fear that he will get caught, Gilles thew the knife there and tried to escape from the mansion. While he was running, Oblongamous caught him. Seeing Oblongamous , Gilles got some idea. He gave Oblongamous a handful of gold coins and asked him to kill Cop for him. Oblongamous agreed and put the coins in his pocket. He broke one of the ice sculptures and took a sharp piece from it. He entered the Lord's room and switched the light off so that even if he get caught, the Lord (Cop) won't identify him. He sneaked right behind Cop and stabbed him on his stomach with the icicle.