A question people continue to ask, decades after his death: how can we take Heidegger seriously as a philosopher, given his outright embrace of Nazism? The instinct to ask the question is the one Jesus referenced in the passage whose citation is the title of the post: When judging prophets, you will know the tree by its fruit.
So if that is right, and it seems right instinctively, poison fruit means a bad tree. But many medicines are poisons if taken in the wrong proportion.
I've been reading Heidegger recently, and some commentaries on him. He is said to be a genius, and if he impressed Hannah Arendt he must have been something like one. On the other hand, I get the strong impression that many of his commentators don't understand what he wanted to say -- not, I mean, that I understand things they have failed to understand, but that I get the sense they are flailing a bit. It is fairly clear that we are still not sure exactly what he meant to say, or why he wanted to say it.
That makes it hard to say just where he went wrong. Philippe Lacoue-Labarthe says it was in being too tied to the organic: favoring the family and therefore the blood, and therefore 'the race,' against the cosmopolitan. But it is possible to err in the other direction too. We often see that from liberal pundits who want to suggest that the US military ought never to be deployed except when it isn't in our interest (for humanitarian reasons, that is, but never because the US has something to gain). It may be that there is a poison here, but it may also be a medicine if you can get it in the right place, and in the right proportion.
Somehow he failed to do that.