I can't say from direct personal experience, but I had a friend reject a Monsanto job offer, explaining that such a first job could become a negative influence on the rest of her career. I'm not in her field, so I don't know what different kinds of employers think.
Hm ok, I was thrown off by the fact that it sounded like you were saying "not many [desirable] companies would hire you after Monsanto". I honestly think your friend might have just been overreacting. Don't get me wrong, I wouldn't work at Monsanto either, but I wouldn't have any illusions about it being a 100% personal decision as opposed to something that would affect my career negatively. My assumption is that most organizations aren't childish enough to turn away employees just because of negative views of their past employers; people take jobs for LOTS of reasons, not simple optimization in one direction (how "good" the company is).
For a facile example, I have a friend who got into environmental law and was very aware that she could either make peanuts defending the environment (at a non-profit) or make bank helping companies destroy it (she has chosen the former so far). If she suddenly had to deal with huge medical costs for a sick kid or something? One would have to be a complete asshole to judge her sight unseen for having a certain company on her resume.
