The evil old white guy is one evil bad guy indeed. He's smart like a fox (due to experience) and made of pure evil. He's usually featured as the main antagonist.
Back hundreds of years ago, European colonial empires would colonize Asian, African, American and Oceanian lands to expand their empires. They would often come barging in, not caring about the population's culture while trying to impose their own. The guy behind this whole scheme was usually an old white guy.
This is still true to this day. Stalin and Hitler were old when they hatched their evil plans. The case of the evil old white guy was first portrayed in movies like Star Wars. General Tarkin is the perfect example of this, giving out cruel orders and not even blinking when doing so.
Of course, this quickly moved to video games, making evil, old, white guys a pretty common main enemy to have in a video game.
What Makes Him So Evil
You would think that an old man would be completely harmless. Wrong. An evil, old, white guy is usually some sort of general. He might not have the strength but makes up for it with the brains. He's old, meaning he's got experience in doing evil things. He'll send countless minions after you and make your progress as difficult as possible.
They're cunning, leaving nothing to chance. And when you think you've outsmarted him, he turns into a giant monster or uses some sort of crazy power to take you out.