Yeah. This is probably the worst GM move one could pull. Of course, at least in this case one could argue that you weren't aware and you could then make it as right as possible or you could go the opposite direction and slaughter the orc children and make a case for them also being inherently evil because of their parents.
Neither is a good situation and the GM is a dick either way.
You are when your GM makes everything you do change part of your alignment. People have even done this for custom maps in Neverwinter Nights, before, under the guise of, "it makes your choices matter!"
Yeah. Sure. Because I definitely should get a lawful point for saying that I'll help the city guard quell a prisoner rebellion, or something, because that's something I 100% do because I'm for law and order of some kind and not at all because I want to protect my hometown and/or family from the degenerates in the prison that want to rape and murder them.