I think the best way to critique is to offer my advice along with a personal revision based off that advice. The personal revision is to be used only to provide extra understanding and additional insight into the advice I attempt to give and not as a template for 'the right way' to tell your story.
As far the first draft I would say good job. There is room for growth but that applies to any person's work.
The first thing I noticed was that the word 'blood' was used quite frequently in the excerpt you provided. I would recommend finding ways to either omit the mention of blood or simply look up synonyms for blood so that the reader doesn't notice how often it was used.
The reader would benefit from small details that help establish setting and personalities of characters. Disregard anything that you plan on addressing at a different point in the story.
Here's the personal revisions I made based off my advice:
The door creaked as he opened it to the study. After the first step into the room, he noticed the floor was wet. Blood covered what used to be hardwood and ornamental rugs. While only a few inches deep, it made him uneasy. It was difficult to see. The only sources of light in the dark study came from dimly lit lanterns, hanging on the sides of the entryway, and a candle burning a top a lone desk. The desk was a rather ornamental desk with nothing on it but a key. Behind the desk was a chair facing the wall behind the desk. "Something's not right." Jack thought to himself. The door Jack previously used to gain entrance into the study was no longer there. In its place was a painting of a little girl between two lanterns. Jack suddenly felt a presence in the room, despite the fact he arrived alone. He slowly turned his head to look over his shoulder.
The door creaked as he opened it to the study. After the first step into the room, he noticed the floor was wet. The entire floor was wet. Blood. Blood covered what used to be hardwood and ornamental rugs. While only a few inches deep, it made him uneasy. It was dark except for the dimly lit lanterns hanging on either side of the entryway and the candles on the lone desk in the study.
< REVISION: It was difficult to see. The only sources of light in the dark study came from dimly lit lanterns, hanging on the sides of the entryway, and a candle burning a top a lone desk.>
The desk was a rather ornamental desk with nothing on it but a key. Behind the desk was a chair facing the wall behind the desk.
"Something's not right." Jack thought to himself. Turning to exit the study, he found that the door he entered through was gone. Just a painting of a little girl in between two lanterns. He slowly turned his head to look over his shoulder.
<REVISION: The entrance door into the study was no longer there. In its place was a painting of a little girl between two lanterns. >
<REVISION: Jack suddenly felt a presence in the room, and slowly turned his head to look over his shoulder.>
Someone was sitting behind the desk. He reached for his pistol and faced the desk. Staring at him was a man wearing a suit. Something wearing a suit. It could not be called a man as it's head was that of a goat with no skin. The skull he was staring at was decaying. Red started to pour from it's eyes. It began to stand and Jack drew his gun. [We don’t know much about Jack right now. He entered a study, which may or may not belong to him, and is faced with an attack from what seems to be a demon. Tiny details may be what keeps a reader engaged. I recommend showing the reasoning for Jack’s behavior to better give the reader an idea of where the story is going. EXAMPLES: Jack reached for his pistol, used to encountering dangerous individuals in his line of work This tells the reader that Jack is in the study on account of business. The study likely does not belong to Jack and the presence of a pistol indicates the line of work has a degree of danger. The study was one small room in what made up the Mansion. The mansion had been abandoned and undisturbed for over a hundred years. Jack was sent by the bank to determine the value of the property. , ] "What the fuck?" Jack yelled, pointing his gun at the demon. The thing began to walk around the desk and slid it's hand to the key, pushing it off into the blood that flooded the office. The demonic being continued towards him. Jack let off 3 rounds in it's chest. The bloody tears gushing from the eye cavities of the skull flowed violently as the skull began to crumble. It's head tumbled back off it's shoulders shattering when it hit the floor. What was left of it, the human body, sank to it's knees and dropped to the pool of blood. It laid there motionless. The body began to disappear as the blood rose higher and higher. "Shit, the key!" Jack ran to the front of the desk and dropped on all fours to look for the key. Blood splashed up into his face and onto his shirt while his hands moved back and forth in the blood and across the floor. Nothing. He looked on the desk.. Nothing. He looked back for the body. Nothing. The door, however, was there again. The blood was rising fast. It was already at his waist. He waded as quickly as possible to the door and placed his hand on the knob. He turned to look at the desk again. The key was centered on top of the desk. The blood spilled over the edges of the desk to the key submerging it. Jack panicked. Already the blood was just below his chest. He kicked off the door and swam for the desk. He dove for the key, his entire body below the surface of the blood. Blindly, he clawed at the top of the desk searching for the key. His fingers hit something. The key. He grabbed it and pushed off the desk, swimming back to the door. He stood up in the blood. It was to his mouth and quickly rose above his eyes as he turned the knob. The door wouldn't open. The blood was above his head. Jack swam up to get a breath before there was nothing but blood in the room. He took a deep breath and went back down. The knob wouldn't turn. It was locked from the outside. Jack struggled to force the door open in the thickness of the blood. It was useless. He needed air. His lungs started convulsing, begging him to breathe. His mouth opened and he sucked blood down his throat. Vigorously shaking the door knob he began to drown. This was it. He tore the gun from his holster, put it to his head, and...
*****I just did a quick re-write of parts I think would serve better to be explained by example on top of my advice. By no means take this as "this is how the story should look like."
[The door creaked as he opened it to the study. After the first step into the room, he noticed the floor was wet. Blood covered what used to be hardwood and ornamental rugs. While only a few inches deep, it made him uneasy.
It was difficult to see. The only sources of light in the dark study came from dimly lit lanterns, hanging on the sides of the entryway, and a candle burning a top a lone desk. The desk was a rather ornamental desk with nothing on it but a key. Behind the desk was a chair facing the wall behind the desk.*
"Something's not right." Jack thought to himself.
The door Jack previously used to gain entrance into the study was no longer there. In its place was a painting of a little girl between two lanterns. Jack suddenly felt a presence in the room, despite the fact he arrived alone. He slowly turned his head to look over his shoulder.]****