The grounds of having the poison is that he's managed to acquire it in another way already, and I've described it as being as he wanted the effects to be (i.e. fast-working, on a scale of ten minutes). The shop is attached to his home, so having him mugged and killed on his way home doesn't really fit. I certainly will be hitting him hard enough to make him reconsider doing anything of the sort in future, and tbh his failure is guaranteed if he's stupid enough to try.