but hadn’t actually hurt Mangione or anybody he cared about, AFAIK
Mangione supposedly had chronic health issues that may have been the basis of hatred for the health insurance business - https://www.washingtonpost.com/business/2024/12/11/luigi-mangione-back-pain-healthcare-shooting/
He had reddit and Goodreads profiles, where he left comments and book reviews which might be used to judge his writing style (also his own handwriting may be analysed, comparing it to that of the manifesto). Although the “manifesto” is very short so maybe not enough language treaits could be extrapolated from it.