According to a press release, the offending content includes the possibility to "sadistically" stab a rat with a combat knife, pick the rat up by its tail and then throw the dead body - "like garbage" - away.
PETA noted that killing of virtual animals can promote brutal behavior and impact the "young, male target audience". There is an ongoing trend in which (German) teenagers torture animals and kill them in a cruel way. According to PETA, the inspiration is delivered by violent movies and video games. The organization also noted that, once a certain level of brutality against animals is accepted, people may be close to be considering violence against other people as well.
The organization asks the developers of the game to avoid such content in future.