Just thought I'd butt in on this convo since I love everything related to Japanese legends since there are much more diverse than anywhere else in the world
First of all, Yokai aren't technically demons or monsters. A more appropriate term would be spirits. They were animals, humans and even items that were turned into such spirits for one reason or another.
And yes, there are stories of Kappas being evil and killing people or drinking their blood. But in the general Japanese folklore, Kappas are mostly seen as benevolent and loyal creatures that share their knowledge with humans.
The majority of Yokai however eat humans, a lot of them using curses to do so due to their inability to attack people directly.
Anyway, my point here is that the Kappas aren't necessarily evil creatures, but were probably added to ACNL due to the fact that they are seen as helpful and knowledgeable.