Ladies and gentlemen, welcome to the eternal boxing match! In this corner, in gleaming white, is Good! Facing off from the other corner, sporting black gloves filled with nails, is Evil! Which will win? A Wrinkle in Time focuses on the fight rather than the outcome, and along the way suggests that good and evil can sometimes look a lot alike. How can one decide whom to trust? In the end, one has to rely on one's gut to tell the difference, because feeling, not reason, makes for more accurate judgment...most of the time.
By making evil an external force, the novel suggests that living beings are intrinsically good if they're not messed with from the outside.
In refusing to explain the origin or goals of the Black Thing, and suggesting that it may be fought through seeking knowledge, the novel portrays evil as fundamentally irrational.