The word “shrewd” more closely captures Satan's character than "cunning." Shrewd means “sharp and clever in a selfish way.” Though “cunning” is not incorrect, “shrewd” has clearer connotation.
To be cunning and shrewd like Satan indicates malevolent brilliance—with the emphasis on malevolent. He is seeking to kill. His cunning is like that of a tiger, silently padding through the jungle with eyes malevolently seeking something to kill and eat.
Consider how clever his tactic was. He subtly made a suggestion rather than an argument to discredit God's authority, casting doubt about God's credibility. Satan asked, "Has God indeed said, 'You shall not eat of every tree of the garden?'"
Through the tone and inflection of his voice, Satan implied that there was doubt that God told them the truth. This is shown by the way Eve replied; she corrected him. She knew from the inflection of his voice that he was really asking a question and casting doubt. When she replied, she over-corrected.
Like a good salesman, the serpent got his victim to agree with him, getting the victim to say “Yes, yes, yes,” and then, "I'll buy it!" Eve was already influenced when she gave her reply because she over-corrected.
Satan successfully magnified God's strictness in her mind, reminding her that the way is narrow. She began to agree with him, thinking about God in terms the serpent wanted her to think. She began to agree, saying “Yes, yes, yes” to the salesman's ploys.
Satan immediately minimized the penalty, saying an outright lie, "You shall not die" (3:4). Then to clinch the sale, he offers her a reward: "You shall be like God" (3:5). What a price she paid! Satan offered a reward that must have seemed so big to Adam and Eve that they could not afford to reject it. What he offered was enough to reorient their lives.
They did not catch the complete significance of what he offered, but enough to know it was big. He offered the self to become the dominating focus of life; "You shall be God." He completely reoriented their lives by turning their focus away from obedience to God toward obedience to the self. He gave them the right to choose and to set the standards of right and wrong. They bought it hook, line, and sinker.
From that point on, mankind has viewed God as a rival and competitor rather than a friend—Someone with whom to compete and outwit rather than cooperate, for they were now gods too!