A few things to note since some people were incorrect.
1. That big demon was a Pitlord not a doomguard.
2. the answer, is clearly obvious and you know even more once you level a Demon Hunter. Demon Hunters use the power of their enemy against them and they are willing to do whatever it takes to protect Azeroth. To other regular people and us heroes, what they are doing is near the same level of what the demons do. They use demonic power and put it inthemselves and have to fight against it and contain that power. Many Illidari also have to sacrifice their own soul for the cause. In this case, if it means they have to fight with the demon within to deal a big blow too the Legion.
So be it.
While the video was short, you got a good idea what they are.