I'm not saying if somebody from the Gl corps is evil, But if it Got into the hands of an evil being WIth monumental willpower (dr.doom, most supervillians) could they effectively use it?

probaly,i mean didnt Sinestro used the green ring with great experience when he turned mad

Why not? Besides the obvious example of Sinestro, Deathstroke overrode Kyle's ring in Identity Crisis and he wasn't even wearing it. They just couldn't kill anybody with it until the prohibition on homicide (sapiencide?) was removed during the Sinestro Corps War.

Sinestro was dictating on Korogur for a while before his ring was taken from him. Now I think that it's a bit harder for them to be evil with the partner system. But if they do evil deeds without the guardians finding out, I think that they'll be fine.

I swear I thought Batman was going to snap or something when he wore the ring. For me Batman is always on the edge, I know he's good, but I think that between he and most other heroes he has the greatest potential for villiany. 
 
Didn't The Penguin have a GL power ring for a short while? Lame.

Probably, the colors of the ring don't have to align with different sides