It isn't too hard to talk them into wars either - especially if you first talk someone into getting themselves killed in an appropriately provocative way. Or even just the right person.

Destroying humanity with mere words seems like a comparatively trivial task from the perspective of "is it even physically possible to do with intelligence?".

Yes, and some people will kill themselves spontaneously even if you don't talk to them (or even especially if you don't talk to them). However, AFAIK there's no generally applicable mechanism that you can use to talk any arbitrary person into killing himself, with a high degree of reliability.

I think it's worthwhile to separate out intentions, plans, actions, and consequences for this definition. If you see memes as intentions or plans, it's odd to see a meme touted as being a consequence ("if you see this bitmap, you will die") rather than an intention or plan that leads to a consequence ("if you slit your wrists, you will die"). The latter obviously exist, the former seem like a definition error.