All trolls are evil or demonic entities that has been forsaken by God to stay forever in the corner of darkness. Due to this reason, they have some utmost grudge against Christian believers that they even have keen sense of smell to easily distinguish them from the non-believers. You can find out more about the scent of a Christian from the Bible on 2 Corinthians 2.
It seems that we can make an speculation to the origin of the trolls basing from the Bible itself. Reading the Bible, it clearly illustrate that most people have sinned by believing into an idol such as Nebuchadnezzar as their god. There are thousands of people or even more who are believers of such idols.
When Moses left his people to travel into the mount, they betrayed him wherein they built a golden calf and altar to worship.
People in the past who used to believe in God or Christ but turns out to be more of an idol believer had been punished wherein they became trolls. This might explain why trolls can clearly smell the scent of a Christian Blood triggering them to cause rampage and tends to be more violent against them. Another interesting aspect to take a look at is that, trolls will turn into stone under the exposure of sunlight. It's like a punishment for them to die and turn into a mere object just like their idol that they use to worship.
Based on the 'Troll Hunter' movie, the camera man was a Christian. Hans, the troll hunter had already warned them about this but due to curiosity of the camera man - he kept silent. Unfortunately, the group got trapped inside a cave consisting of huge specie of trolls. Everything should all just be fine where all they just have to do is simply wait for the perfect opportunity to escape but then, the camera man had admitted that he was a Christian.
Hans told the group that they had to find a way to escape out because the trolls will soon be able to smell the Christian blood out of their camera man. Thus, they waited for the perfect opportunity to sprint their way out of the cave. As they got the chance, the crew was able to run out of the cave except for the camera man who was caught and dragged back into the troll's cave. The camera man was then replaced by a female Muslim.
The photo shown above is a statue of a troll and its called 'Troll Smelling Christian Blood'. It was made by a sculptor named 'Niels Hansen Jacobsen' in 1901 and it has caused quite a controversy in Copenhagen. This is due to the reason that the troll statue has been placed in front of the 'Church of Jesus' in Valby.
Today, the statue has been moved and currently standing at Carl Jacobsen's museum, Ny Carlsberg Glyptoteket. In 2002, a copy of the troll statue has been created wherein it was placed right in-front of the church where the original statue used to stand. This was to celebrate the 100th birthday of the troll statue that has been forgotten how controversial it was before. Another copy can be found at Vejen which became a part of the Troll Fountain.