One of the things I don't understand is not one single person in the group even remotely mentions to Herschel the fact that they were at the CDC and they pretty much knew what the "disease" is/does.
Herschel apparently believes that it's possible to cure people who are suffering mortal wounds, fatal blood loss, and their organs hanging out. And he believes all this despite being a doctor. (Yes, I know what kind.) His medical knowledge should inform him what is and isn't possible.
It doesn't matter what you tell him. Denial isn't just a river in Egypt.
His medical knowledge would also tell him that the dead don't just rise from the dead and eat living people. But there's the new reality, that science would have said was ridiculous.
Here's what i think Hershel saw, that ingrained in him.
Remember the Shane flashbacks where soldiers were killing people who weren't walkers yet, but were infected?
I bet what he saw were some viral videos where we saw soldiers who had gone off the deep end, and lost all of their sympathy and callously (perhaps with an evil glee) shot people who had the virus but still had moments of lucidity to let the viewing world, that at that moment, they were STILL human. Shooting them in the head at that point might have been "practical", but it was still inhuman in how they addressed those people
I also remember Jim...they "should" have shot him in the had, but he was still human enough to say goodbye and let him be. That's probably how most of us would treat infected people, especially those we know.
AIDS was similar to the zombie plague in the beginning -- when you had the virus, it WAS a death sentence. But drugs were developed so that people like Magic Johnson could celebrate 20 years of living despite the death sentence.
How unrealistic would be for Hershel to have that same hope in mind?
Again, as others mentioned, he hadn't seen some of the realities of the Walkers before (i.e. shooting them and them NOT going down).