The shadowy being is actually trying to help, but he appears to be lying because for him, everything is reversed.
Note: here there be spoilers!
The shadowy benefactor does not actually seem intent on killing you. They seem quite content on simply keeping you captive and living with you for all eternity. He even gives some true tips that help you make it to the real ending. Then why the murderous lying tips during the course of the game? They are actually trying to keep you alive, but since they are a mirror version of you, their world is inverted.
It has at least been established that the spikes actually hurt the shadow. It would make sense that, perhaps, arrow keys for them are movement and space bar is jump. Beams of light reset them. Spikes, as shown, cripple them, they die to bottomless pits, they can kill shadow beings by jumping on them, they can touch and use the gems, the boots powerup for them is high jump, and so on.
(Actually, come to think about it, he dies to the gems too... but maybe that was an effect of the spike as well?)
- Possibly untrue — the benefactor often snickers after giving you deadly advice if you look at their radio face (especially after the "See? You can trust me..." line), implying that they really are trying to kill the main character.
Because there's really no other way to explain the weirdness happening here. The protagonist is trying to break free from their own insanity. That explains the Mind Screw
happening and why it stops with the defeat of the shadow.
The "benefactor" really is the main character's Shadow
It has just gone berserk and is trying to kill MC because MC refuses to accept it. The reason it's more subtle about it is it hails from the Hermit, which is related to intelligence and in the reverse position can mean, you guessed it, deceit.