It is a terrible show for anyone not grounded in solid doctrine of angels, demons, Heaven, Hell and, most importantly, the nature of God and Man.
The mythology of the series is fantasy, an unreality. Nothing they say has any bearing on spiritual realities. If one can approach the show with the proper understand that it is all pure myth, then this particular problem is not relevant. The problem is that such fantasies that are close to reality, are the ones that most easily skew our thinking.
The violence and gore in the movie is not only a pause for concern, it also serves as an analogy of the above. If one understands the nature of such devices, and is not tempted into violence, then it is might be okay. The same problem exists for the sexual promiscuity.
That being said, the show exhorts such values as courage, love and duty.