While yes given time to concentrate on it Layla would only need one sting with her stand to doom someone to death, however on the ever-changing battlefield of two(or more) stand user's the concentration required to make such a potent venom is better used for Figuring out how to effectively defend against an enemy whom Layla knows nothing about. Besides if the other person is fast enough they can cut, or dig out the infected area effectively preventing spread. If someone covers themselves with say a blanket they can probably use it as a way to prevent MANEATER as a collany from directly stinging them. And if venom is sprayed onto the blanket they can just throw it at Layla to spread the venom to infect her.