What a villain is like depends on the story/creator.
I personally don't really enjoy the cackling mad, woe-unto-the-world-for-I-am-eeeeevil type of villain - unless the entire story is set up to the overblown and melodramatic, in which case it fits. But insert that kind of villain in a story where everybody else has realistic motivations? Nope.
I just... I want to be able to understand why characters do what they do, and the same goes for villains. Nobody wakes up in the morning and things "Today, I'm going to be evil!". No, everybody wakes up in the morning thinking they're doing the right thing - or are at least able to justify to themselves why they aren't. They do what they do to achieve a goal, and that goal is rarely "let's be evil". Everyone's the hero of their own story.
A story with the villain as the main character would definitely work, but the creator would have to work hard to get it across to the reader - as readers, we tend to assume the main character is the good guy, and letting the readers know that they aren't can be a challenge.
As for your last question... I wish I could tell you about Grassblades' main villain, but I can't! XD It might spoil the story, since they haven't really shown up yet! So far, the main antagonists have been a.) a bunch of hapeless bandits, b.) a thing with a snake tongue and terrible teeth, and b.) Masahiro's own horrendously bad mood.