What Schulz is saying here is that individual people fall short of their potential as people. Schulz is saying that he loves the idea of mankind, which is the idea that people have a great deal of potential. But the problem is that individual people have all these flaws that make them fall short of what people can be.
We all love humankind. Humans are capable of wonderful things. However, each person as an individual has multiple flaws. Each of us does things that are so much less than what we could be. We ignore our consciences and commit acts that are sinful or wrong. By doing so, we fall short of what we are supposed to be.
So Schulz is saying that humankind as a whole has all this potential and he loves that. But individual people fall short of this ideal and he can't stand seeing that happen.
This quote falls in the same category as saying someone "couldn't see the forest for the trees." Someone in that situation sees so many individual problems (separate trees) that s/he is unable to see the overall picture (the forest).
Schulz is saying that he loves and appreciates humanity as a whole, referring to the human race in general. However, his statement holds that he does not like people--specific individual members of the human race as individuals. Schulz is capable of seeing and loving mankind in general, but can't stand individuals within that group.