That's a horrible use for propane and propane accessories.
If arsonists were known to roam in your neighborhood, wouldn't you call the police if you saw someone wandering with a gasoline tank?
THe rule of comepting harms would say you are doing the right thing, but of course you know better, you would not.
In case you have difficulties putting it together: BB balls + propane + teddy = hidden nail/BB bomb.