I had a yandere girlfriend(lol) in highschool, granted she was batshit insane I actually enjoyed going out with her till she became.. well.. batshit insane. Trust me, getting asked about who you were with every single day, plus being stalked and being incredibly paranoid by it is not a fun thing to do on a daily basis. Throw in the fact that she sometimes talks about things you weren't even considering yet in a very serious tone(children, marriage, etc.) and you have yourself a yandere who is willing to kill girls that touch you. This girl got into a physical fight with one of my best friends just because she gave me a ride home.
Also, just for the record, Kotonoha is not yandere. She became mentally unstable because of Makoto, which led her to do what she did. I can say the same for Kaede as well, since Rin is a asshole, but she also was creepy as shit as a kid.