With the Slayer Scythe it makes sense. It was specifically fashioned for Slayers and a slayers primary enemy are vampires. I can understand their building in a slayer preference/vampire repellent quality.

But Trolls aren't necessarily 'good' creatures. Olaf was wanting to eat innocent babies. So I'm not sure why a Slayer would necessarily have more mystical affinity to it than any other supernatural entity.