It's interesting watching people rush to label the Minnesota shooter as this or that. But some facts. The so called manifesto is actually a 4 page suicide note. The writing on the gun is a very specific kind of cultural script, and a claim to belong to a chain of killers.
The notebook in Cyrillic is probably just coded to hide the content from nosey family, and is unlikely to be an indication of a link to Russia. All in all? A tragedy for all involved, both the murderer, their family, and the victims.
I haven't seen any details of their Internet history, but I'll unhappily assume they were in an online community dedicated to serial and mass killers, and had collected a lot of gore. If they fit the stereotype, their will also be all kinds of sexual and other objectionable material.