Being a singer myself, and someone has already mentioned it, it is really important to note that singing is an organic instrument. EVERYONE has nights and days where they can't do shit. Its just the way it goes. He could have been incredibly sick, or just had an off day... who knows. Sometimes people will advise you not to talk since that can make matters worse. Continuing to sing the easier parts properly, without straining, is the best that probably could be done in that situation. I just know that Tool takes a lot of pride in their live show, and it doesn't seem like they would purposely make the show less than their high standard.

I'm not sure if hatred or anger towards them is the proper response.

I do wish that they could have had someone come out and briefly state what was going on, though. That would have made things better for everyone.