For Radovid, they had to stick closer to the books in order to make a certain events (that were already foreshadowed in the final novel) happen and his short appearance in the Lady of the Lake left no doubts that he was a bastard. In the previous games there were moments when he displayed his petty nature, but in the final game he just completely snapped due to all the pressure, sacrifices and paranoia he suffered through.
It's actually pretty funny to see a disparity between polish and international forum on the matter. People on the former where first introduced to Radovid via books, so since he appeared in the games, people basically awaited him finally going crazy and showing his true colors. People in the international forum knew him mostly from the games and read the books later, so they used to a certain image of Radovid, that was completely destroyed in this game. It makes me think that it was somewhat intentional on the RED's part to make him kinda likable in the first two games and then show what kind of jerk he truly is to make his actions more shocking.
So my final word on the subject is that you may not like this approach, but in my opinion it was just necessary for the sake of staying true to the story depicted in the original books.
About Eredin, I always saw him as a nothing more then manipulative jerk, so I didn't really care about him.
It's not that he suddenly became mad, but the fact that his "war genius" was never shown, also his damn chess scene was so badly written it did not make any sense, as for Eredin, he is a manipulative jerk, but the Witcher 3 Eredin is a joke.