They killed an innocent man in order to advance their position in the King's eyes and to give the Prince pause; I realize that the influence the Prince wielded needed to be circumvented somehow, but as they said, it was done at a very high cost, and was a foolish enterprise at best.
As I stated previously, fighting for other than self is a higher calling, and I give them recognition for that, but killing the large redhead was not the way I would have done it; seeing as his sword was of greater worth than the Prince's, I would've taken the Prince in the dead of night, bound and gagged him and left him as a sacrifice for the Wendols- to me, that would've been just as the Prince wasn't necessarily concerned about the people, only in being king. For the greater good, of course.