Yes it is. The words بهتاناً and إثماً are both Hal حال and they are in the accusative case. Another case (less likely, but still acceptable) is that both are "causative adverbs" مفعول لأجله (مفعول له). This arises from the fact that these two words can be the answers for "how is the state of the doers?", and "why are the doers doing this?" How is the state of these people when they are taking that thing? They were taking it with false accusation and with great sin. -- Here the two words act as حال because they describe "how". Why are they taking that thing? Because they are slanderers (falsely accusing others) and wrongdoers. In other words, nobody can accept taking it unless he is committing a sin. -- Here the two words act as مفعول له because they describe "why" (their motivations).