The Case of a bottle of cyanide.(氰化钾)

Arthur Maxim sagged in an easy chair.
His right hand lay on his lap, clutched
about a bottle of cyanide.
“He’s been dead about fifteen minutes,”
Dr. Haledjian told Carter. “Have you called
the police?”
“Yes---but I called you first.
I knew you were at the hotel.
Arthur was depressed, but
---good heavens!---I never dreamed he’d
kill himself!”
“You knew him well?” asked Haledjian.
“We were kids together,” replied Carter.
“Since his wife divorced him, he’s been
despondent (沮丧的,消沉的). I suggested
this vacation. He seemed better yesterday.
We played golf and fished.
“Half an hour ago I went down to the lobby
for a newspaper. Just as I returned, he
drank that bottle in his hand.”
Inspector Winters arrived. Haledjian
summed up his findings.
“You’ll notice how lax the body is,”
said the sleuth(侦探), pointing to the
drooping position the corpse had assumed
in the chair. “Cyanide kills instantly,
and the muscles go limp.”
“Maxim appears to have committed suicide
by drinking cyanide. But no conclusion
is possible till we know for certain
the cause of death.”
Two days later the inspector told Haledjian: “The lad confirms that Maxim’s death was
due to cyanide.”
“That clinches it!” was the reply.
“Arrest Carter on suspicion of
murder!”

Why?