Were aristocrats people who took power illegal way?

No. Aristocrats were officially appointed by the King in their offices and their powers were often described and limited by law. They were powerful mostly until the late 19th and early 20th century, when the majority of the population in most countries got the right to vote. So they did not 'take away' power from anyone. They exercised it in an age when the common people did not yet have political rights.