While koyashaka has drawn a fair amount of yandere images in this style, the other works have some indication of love (such as hearts, 大好き wall-of-text, etc) to go with the murderous intent. There are no pixiv tags indicating it as being yandere, so it's uncertain that's the cause for poor Flan's unhappiness and violence. Thus, I'm refraining from adding that tag, even though it's likely she's been taking love lessons from Koishi (see post #1534383).
