Citron Research's Andrew Left, who has shorted Tesla shares, told CNBC on Thursday he is surprised that shareholders of the company reacted so strongly to the SEC's fraud charge against CEO Elon Musk.

Shares of the electric car company fell about 13 percent in extended trading after the Securities and Exchange Commission filed a complaint alleging Musk issued "false and misleading" statements and failed to properly notify regulators of material company events.

"Of course he's going to get charged by the SEC. If you lie, you get charged by the SEC. To me, that was obvious," said Left, a well-known short seller.

"For the market to react like this, it only shows that they think a halo existed, that no one is above the law," he told "Fast Money."

In August, Musk tweeted that he was considering taking Tesla private, adding "funding secured."

MUSK TWEET

"When he made that statement, it was not true. The SEC had to charge him," Left said. "If not … you couldn't be a short seller. Every CEO would just come out and say tomorrow they were thinking of taking their company private."

Musk later explained that he had been in discussions with the Saudi Arabian sovereign wealth fund and felt confident the funding would come through at his proposed price of $420 per share.

After the SEC's complaint, Musk called the allegations "unjustified."

"I have always taken action in the best interests of truth, transparency and investors. Integrity is the most important value in my life and the facts will show I never compromised this in any way," Musk said in a statement to CNBC.

With this latest news, Left said the stock is a "difficult short," but he would not discuss whether he was now trying to cover his short.

"I wonder now if Elon Musk gets removed as CEO — is that a positive or is that a negative for the company. Who knows right now?" he said.

Early this month, Left filed a lawsuit against Tesla and Musk alleging that Musk manipulated the price of Tesla shares by issuing materially false and misleading information.

—CNBC's Sara Salinas contributed to this report.