With a name like Mister Sinister you don't expect him to be a nice guy, but the guy is so depraved that his actions have pushed the boundary of what is acceptable for publication in a comic book.
He was born Nathaniel Essex, a scientist and contemporary of Charles Darwin so obsessed with human evolution he conducted experiments on his disabled son- and after his son died he dug up the body so he could continue experimenting on the boy.
His wife dubbed him "Sinister" with her dying breath, but she had no idea how cruel Mr. Sinister would become as he kept tinkering with his own DNA and the DNA of many other mutants.
This tinkering would result in the creation of Jean Grey and Namor clones, the birth of Cable and the Morlocks (who were products of Mr. Sinister's former student), and the deaths of many people.
Mister Sinister continues to play God, which is why he makes such an interesting enemy for the X-Men, who have proven to be the only superfolks who stand a chance against Sinister.