If Monsanto want their patent to cover future generations of seeds derived from a Monsanto seed. Would they also claim liability if one of those generations had a mutation that caused the plant to be toxic and kill people?
Of course not, that's easy, they would fight all accusations of liability in that case. The banning of GM crops is already starting around the world and I'm sure companies are fighting this "unfair characterization" of their product.
No, they would sue the people who are dying, because they would be illegally benefiting from Monsanto's product without paying for it.