The victim:
a)Marisa [ ]
b)Patchouli [ ]

Noo... My koakuma turned into a yandere i_i' ! What happened?

She looks betrayed. Like some one lied to her and broke her heart.

FRien said:
Noo... My koakuma turned into a yandere i_i' ! What happened?

I think she just finished organizing every book in the library and upon looking at her grand achievement Marisa and/or Reimu burst in and manage to remove every book from the shelf. Patchy confronted them, Koakuma snaps smacking Patchy out of the way to confront the culprits.

Maybe that happened

FRien said:
Noo... My koakuma turned into a yandere i_i' ! What happened?

She was getting frustrated from all that boring "loyal Koakuma" portrayals... probably.

Judging from the artist tags, I think the victim was Patchouli.

[]'s tags said:
上司(パチェ)に愛想が尽きた顔
こあ「日記・・・読みましたね?」

or

A face whose courtesies towards the boss (Patchy) have run out
Koa 「You...read my diary, didn't you?」

Looks like Patchy coughed in it while reading, too.

Unexpectedly evil ? come on guys, she is a koAKUMA

she looks adorable :D

FRien said:
Noo... My koakuma turned into a yandere i_i' ! What happened?

Someone will die.

ExDragonMaster said:

A face whose courtesies towards the boss (Patchy) have run out
Koa 「You...read my diary, didn't you?」

Heh. 愛想が尽きる is an expression, meaning "to be disgusted with; to be fed up with; to run out of patience with". You better not break it up into separate words, because "courtesies running out" sounds horrible.