[yyyk]

>Plastics don't self-manufacture. You might not be able to control the rate.

The rate is limited by the process. Since no precursors exist, it must 'self-manufacture' from scratch. This has inherent limits even before introducing competition for food, poison, predators that eat you even despite them not being able to really digest, etc.

>Just because you kill something doesn't mean you break down its carbohydrates. Reverse chiral organism skeletons could bioaccumulate and we could have a situation similar to the Carboniferous.

So you don't break it down. Nature will have plenty of time to adapt. Humans will step in if needed.

>Someone might be able to synthesize a bacteria in the lab given enough time and effort from an organism that proliferates quickly.

That's an incredibly messy way - create an entire L-chiral biochemistery - to get a weapon which doesn't have a setting between 'kill everything' and 'do rather little' (IMHO, the second being much likelier). There are far worse and more directed things one can do with a lab. Even the absurd 'kill everything' goal is far more likely to be reached in different ways.