The article you linked does not support the claim you made.

It argues that antidepressants may be associated with aggression or violent behavior in a small susceptible subset. That is very different from “SSRIs explain the rise of school shootings.”

The “most school shooters were on SSRIS” claim has been studied directly: https://pubmed.ncbi.nlm.nih.gov/31513302/

Their conclusion: “most school shooters were not previously treated with psychotropic medications - and even when they were, no direct or causal association was found.”