Evil? what is this concept you speak of, there is no morality here just what we choose to do.
Shades watches the yeti for a while then thinking lets give evolution a helping hand here, taking one of the tiny virus types from the ecosystem alters it to feed off either it's host or violence caused by it's host, slowly kill the least violent ones.
Create a virus to weed out the passive yetis
After watching the virus slowly spread he realised it was still too vulnerable to cures and so once again starts tinkering with it, forcing it to mutate rapidly with each generation but keeping the core of it's goal the same. Watching it flourish to countless different varieties.
Exponentially increase the strains of the virus
Generations pass as Shades watches the virus explode among the population. But still there is risk and so taking a single strain he alters it to have two stages, the first lays dormant barely feeding, just taking enough from it's host to slowly spread through the population. The second kicks in when a host is not effected by any of the other virus where it rapidly multiplies killing within days and spreading quickly.
Create a dormant lethal strain and link the trigger to encountering non-infected yetis
Shades releases this strain back into the population where its quickly lost among the others.
I'm amoral not evil, what do the lives of mortals matter to us anyway?