T. Kaczynski has published 6 papers in 4 years (1965-69) which can be considered as a good beginning of a promising career. According to Mathscinet database, one of his papers was cited 4 times (2 of them by himself), and three were cited one time each (two by himself). Mathscinet does not count all citations but most of them.

I am somewhat familiar with the subject of his work. I would not call it "brilliant", but qualify it as average PhD graduating from a good university.

Remark. That two well known serial killers among mathematicians were both doing complex variables, is a coincidence, on my opinion:-) (The other one was Andre Bloch who was a really brilliant mathematician, but unfortunately, insane.)