And why wouldn't you condemn grandparent post first?

https://news.ycombinator.com/item?id=47439266

GP reported on an opinion and called it insane. That's not an ad hominem.

There is a difference between judging an opinion and judging a person. If the response had been something like "what is crazy is to think the world can just switch off its dependence on petrol suddenly", I would not have reacted either.