Yeah, I'd have to imagine there would not be a lot of people left after not so long. Small isolated groups who kill anyone they meet, because better to kill than be killed. Dynamics would also depend on how hard you have to think, how long it takes to kill, and if anyone would know if it was you or could tell you were thinking someone dead while you were doing it. If everyone carried guns, for example, we would all be wiped out because anyone could see you killing another person and be able to react accordingly. Being able to kill without consequences, from the comfort of your living room for example, would be catastrophic.
Once long ago I read a short story about an altruistic alien people who exchanged this dull silver metal with each other for commerce and everyone had enough and everyone was happy. Then one day a greedy alien was born and he started hoarding as much silver metal as he could into a pile. Then the plutonium reached critical mass, blew him to smithereens, wiped out his entire town and the cycle started anew with no one remembering that their coins exploded when you had too many of them and greed never evolved. Questionable physics, questionable morality, amusing thought experiment. Wish I could find the story to link it, but I've already given away the joke.