The phrase who "ostensibly sought to expose economic injustice" indicates they wanted to outwardly appear as seeking "to expose economic injustice". This implies that reforming society was not their real agenda. Thus the word for the first blank is disingenuous. That is insincere and hypocritical. The claims to a radical agenda were disingenuous. And they were meant to give lurid thrillers an appearance of respectability or nobility. The word for the second blank is high-minded purpose.

A lurid thriller is a thriller that is shocking and sensational. Since the authors were disingenuous in their claims, we need the opposite of lurid, something that is serious and noble.