What's the fallacy in the following argument: Cancer cells can be eliminated by treatment. Aren't criminals cancer cells of society? Eliminate criminals.
A second fallacy lies in the grammar. The answer "eliminate criminals" ignores the prepositional phrase of the statement "by treatment." Elimination is dependent upon the treatment. Really the original statement should be reworded: Treatment can eliminate cancer cells. Then, if the analogy of criminals being the cancer cells of society is accepted (false analogy, though, they don't share enough qualities in commone), then the conclusion would be "Treat criminals", not "Eliminate".
Well, in many case more than one fallacy applies, but this one seems pretty clearly to be a weak analogy. Simply put, criminals are not the cancer cells of society, and the parallel is a weak one, leading to, as the final phrase indicates, false conclusions.
Looking a bit outside the statement, consider how cancer (cells) is treated (chemotherapy, radiation therapy, surgical removal of the cancer, etc) and the cost of going through treatment. Cancer treatments not only eliminate the cancer cells, but they also kill other cells (good cells) in your body. So if criminals of society were like cancer cells, treating/eliminating them would mean killing other innocent people of society at the same time. Not to mention, cancer treatments are not always successful (person dies, cancer cells come back, cancer cells aren't 100% eradicated, etc). Like what others have said before, the analogy is weak or can't be made between the two. In my opinion, I think one difference lies in the fact that for cancer patients, if they don't go through treatment - they die, if they do - they might still die, but at least there's chance of living. And thats not necessarily the case for criminals in society, its not a do or die situation.