How do you kill a man locked in a room?

A thin man lies half sprawled across a large wooden desk. He gently holds a pen. On the desk there are the man’s glasses, a photo album, and a piece of paper next to a stack of more paper. On the single page there is written:
I was born in 1939 in a small town outside Seattle, Washington. My father w-
The page ends in a long scrawled line.
Behind him, there is a bookcase. On his left there are three long windows each adorned with red velvet drapes and iron bars. To his right, there is a fireplace and above that a massive self portrait. If the man were alive, he would have seen in front of him two of the largest South Bay policemen breaking down his large wooden door. Until that moment, the office had been kept pristine.

“This is the General’s private study. He would come here to be alone. He only had one key made for this room.” The butler took a long pause and looked at the ground.
“Go on.” Detective May scribbled furiously.
“The last time I saw him was just last night, 9 hours ago. He checked the house as he always does… (sigh) as he always did. I was never allowed into the room.”
“Yeah, ok that’s all I need for now. Please wait outside. Thank you.”

May had come across scenes like this before. Usually the police don’t stick around too long for corpses without bullet holes, but this man was a government official. Theses cases always required full reports.
“Damn” he whispered under his breath. His pen had run out. “And now here comes that muscle bound idiot.”
Gregory Brig, Captain of the South Bay Police Department, strolled up to May.
“Looks like that old General finally died. Stupid old man, he started his biography too late!” Brig chuckled to himself.
“You are unbearable” May mumbled
“Shut up May. What did the doctor say about the General?”
May quickly flipped through his notebook. “The doctor said the General died ‘around 8 hours ago of a heart attack or some heart condition.’ He will check in the autopsy, but he is certain it was all ‘natural causes.’ I think he’s wrong”
“Uh-huh” Captain Brig had wandered to the window “It’s been a shitty winter, eh, May?”

May ignored the Captain and patted himself for another pen. Something was still bothering him. He just needed to gather his thoughts. He was about to ask Brig for a pen, but remembered that he was an imbecile cop. “Only imbeciles cops walk around without pens.” he muttered.
“What was that?” Brig had wandered over to the window.
“Oh nothing, Captain” he turned to one of the deputies. “Did you find that key yet? It should be in his left pant pocket.”
The deputy shoved his hand into the Generals pocket. “It’s here sir! Ha, how did you know?”
“The General was left handed. Look where the pen is.” May smiled and quickly went to the desk. “I don’t think he’ll mind. Nice pen.” he mumbled.
May pried the silver pen from the General’s hand. It was probably a gift from another official. The pen was wonderfully engraved with the General’s initials. As he admired the weight and balance of the pen, he clicked it open. He froze.
“Hey lemme see those keys!” The deputy tossed them to May. He inspected them closely and found a small piece of tape attached to the leather flap. He then moved to the corpse and felt the neck. “Hey Brig, we’ve got a murder here!”
“That’s impossible. There’s no way!”
“Just go get that Butler.”
Brig left the room and returned with the Butler. May stepped forward with notebook and pen in hand. He smiled.
“You are real outdoorsmen aren’t you?”
“Uhm…I suppose I was”
“You go fishing a lot?”
“I used to before I worked for the General. I went with…”
May began scribbling in his notebook. “It’s always the Butler” he mumbled.