A GMO can easily be made to be as toxic to as any pesticides. So think DDT, 2,4-D, Chlordane, Agent Orange, Heptachlor and all that.
To be technologically precise, one could not literally put these molecules in a plants' genome, but other similarly dangerous molecules are quite feasible.
Presumably, pesticide can be studied carefully, scientifically, and deployed under the supervision of an agriculture engineer who will express utmost respect for the health of their neighbor, and for the environment that supports all life including humanity's. In practice, that's not what happened.
Pesticides were applied indiscriminately, with wanton abandon and callous disregard for safety. Farm employees died, cancer rates exploded. It was decades after the publication of Silent Spring, the birth of the modern environmental movement, and massive million-people protests throughout the country that national politics began to take notice of the issue. Only then were somewhat effective regulation put in place.
So given the history here, it seems justifiable to expect large agriculture companies to behave with as little care for the health of the broader community as they displayed back then. Similarly, American politicians could be expected to be as laggard as they were back then, or perhaps worse in the eyes of anyone who believes this political system has been switching from democracy to plutocracy.
In sum, like all other novel technologies with potential for both profit and harm --medicine, cars, planes, rollercoaster rides, bodypiercings, etc-- GMOs raise questions of trust, regulation, enforcement, transparency, and struggles of interests. Technologists like myself and yourself would much prefer to see technology used wisely as a matter of course. Alas, it cannot be the case, and thus GMOs necessarily have a political dimension.
Given this, the question we should be asking is: what are the key features of the body of law that regulates the safety of GMOs, and can these be trusted to be effective?
Here with hit a wall. It is nearly impossible to know. Let's out me as the rather obsessive readers of news that I am. I can speak about the FDA drug enforcement rules, the FCC's, the EPA, the WHO, the Feds', the Dodd–Frank Wall Street Reform and Consumer Protection Act. I could probably fill a page or two with details of how Obamacare work and why it works. I love watching crash test dummies videos; I listened to the all the Plane Crash Engineering Investigation episodes I could get my hand on. And yet, I have no clue about GMO regulation.
I challenge this reader to find a long-form piece in any national American newspaper, let's say between 1990 and now, that explains how this country establishes the safety of a GMO in a legally binding manner, and what penalties are scheduled for abuses. Who introduced the bill and who paid for their reelection campaign? What were the different schemes considered? How do American GMO safety regulations compare with those abroad? Are they better, worst, same? Nobody knows.
American science journalists have done a pitiful job of providing transparency to the situation. Now that it's 2015, there is nobody left to do it since nearly all science journalists at large newspapers were fired when Craiglist cannibalized the papers' ad revenues. A well-earned popular distrust of GMOs grew into that vacuum and it's just a sad situation all around.