Let's look at the details exactly as Connell writes them so we can uncover words that might foreshadow the truth about his nature, which we know to be a cold-hearted killer who kills for sport, and to relieve himself from boredom:
In a cultivated voice marked by a slight accent that gave it added precision and deliberateness, he said, "It is a very great pleasure and honor to welcome Mr. Sanger Rainsford, the celebrated hunter, to my home."
... Rainsford's first impression was that the man was singularly handsome; his second was that there was an original, almost bizarre quality about the general's face. He was a tall man past middle age, for his hair was a vivid white; but his thick eyebrows and pointed military mustache were as black as the night from which Rainsford had come. His eyes, too, were black and very bright. He had high cheekbones, a sharpcut nose, a spare, dark face--the face of a man used to giving orders, the face of an aristocrat.
There are many descriptors in here which echo the necessities of a highly successful hunter. Look at the words I have italicized for you.
Rainsford also notes that this man seems bizarre. It is almost as if Rainsford wonders if this man were capable of great strange things. Look at the bold. We believe we can see much in the way of a person's motive and morals by looking them in the face.
To add, we also look to the eyes. Closer looks at his face reveal great black and dark features. Look at what I have bolded and italicized for you. Dark and black colors are mentioned 3 times. This signifies evil.
Finally, the comments about him as in control of himself, a military figure, and one who commands others further demonstrate his control of the situations in which he partakes.