An "evil grin" is a specific type of emoticon or visual symbolizing a specific type of smile, a face with a wide grin and, typically, downturned eyebrows, signifying evil intent.
Techopedia explains Evil Grin (EG)
Many different kinds of these emoticons can be rendered as GIFs or small image files, in ASCII characters, or in specialized visual icons generated by keystrokes.
The evil grin is one emoticon that can be rendered in text characters. One common way is to use a greater than sign, a colon and a right parenthesis in sequence: >:)
Another variation is to replace the colon with a dash: >-) which represents a face with slanted eyes instead of slanted eyebrows.
Emoticons such as the evil grin have been used across many different kinds of platforms, in instant messaging, emails, on intranets, and across the global Internet to signify different types of emotions and expressions by users.
Many of the simple ASCII emoticons used in the dawning years of the Internet have been replaced with standard icon visuals for today's more advanced web browser graphics environments.