Evidence: There's a chance Andy has dissociative identity disorder, according to Reddit user u/KANNABULL. We know about his history with anger issues and his attempts to manipulate people into liking him. His pattern of behavior is most noticeable in Season 3, during which Andy seems to have over-the-top responses to small situations. However, in the following season after his anger management training, it seems as though Andy's softer side has taken control; when Angela treats him badly, he remains a kind and patient fiancée. Neither of these emotional reactions are normal; Andy is either far too kind or far too aggressive, indicating that he has two personalities on opposite sides of the spectrum that he has difficulty balancing.
The user also provides physical evidence that suggests that Andy may be the strangler:
- In the episode "Did I Stutter?" Andy is willing to sell a perfectly good car for a low price, possibly because he's trying to get rid of evidence.
- In "Job Fair," Andy's hands are damaged from blisters which he supposedly got from playing golf — however, he could've worn golf gloves to prevent this. Perhaps he damaged his hands another way?
- In "Weight Loss Part 1," we find out that Jan's candles are doing well because of a vigil for a missing girl. Which character briefly dated a high-school student, then got mad because she lied about her age ("Product Recall")?
- In "The Duel," Andy's reaction when Michael Scott tells him that Angela cheated on him is subdued, possibly because he was afraid that Michael Scott found out his real secret — he's the strangler.
- Also, we find out Andy has invested in a quiet car, which he later states would be helpful for escaping the police ("Viewing Party").