Fist O Cthulhu’s guide does the best at explaining how the points work and what is required for the achievement.
Some people are saying that after a recent update it’s now only required to kill everyone, but after about 3 games of killing all survivors and not getting the achievement that doesn’t seem to be the case. The points requirement is still needed for the merciless killer grading.
I’ve made a video of the game that finally got me the achievement of anyone would like to see how to play (poorly) as the trapper.