Can we make a theoretical weapon?
Like, can we kill someone with a theory? A legitimate infohazard?
Do you mean indirectly as wierd joked, or directly via a sort of memetic basilisk (that is, as opposed to the classic
Langford basilisk, which uses an image as a vector)? I don't think such a thing is actually possible. It seems rather implausible to me that we can create something created via a mental vector will result in a destructive positive chain reaction that kills the person with a literal thought. That's not to say we can't introduce fresh new vulnerabilities in the future; hacking someone's "wetware" through a digital MMI is a time-honored cyberpunk tradition, after all, and anyone familiar with the damage malicious computer access can cause in the modern world should be aware of the potential issues that could be caused should a person of a particularly malicious bent obtain root access to, say, a
wifi-capable pacemaker. Leap into a future with digital memory implants or processing-enhancers wired directly into the brain, and I could see these being used as points of access to cause all sorts of problems. I think most hypothetical variations of the concept in fiction, apropos of typical delivery mechanisms likely in reality, tend to be either visual (a la epilepsy) or aural (such as the much reputed, but rather unproven "brown note"), however, unless you count jokes about mental instability in key figures of mathematics being indicative of the subject's effect on its scholars; Lovecraftian-style things of which one was not meant to know tend to drive one insane rather than kill one directly.