Gosh, in today's world, and really from even my own childhood, guns and knives were pretty common things boys liked. We played cops/robbers and cowboys/ indians. We had fake guns and knives. Nerf makes a bundle annually over guns and swords for kids to play with. Boy scouts offers merit badges in skills related to both of these. They can become cool hobbies. No. Does NOT make someone a killer or derelect for this interest.