Question: Do animals who kill other animals create negative karma, or are they ‘excused’?
Answer: Animals might kill with less actual viciousness; with more instinctive violence for food. (However, some animals like cats do catch and kill out of curiosity too, in a playful but deluded way.) When there is less intention during killing arising from hatred, which is the usual cause of negative karma from killing, there will be less of such karma created. However, it is likely that there is still some habitual hatred at work.
As karma is a natural law that applies to all sentient beings, no one is excused from its workings. This is one of the reasons why it is difficult for beings who have fallen into the lower realms to rise to higher realms, taking a long time, both for much negative karma that caused the fall to exhaust, and for change of ways. (It can be due to killing of animals and living in an animal-like way that caused fall to be reborn as animals in the first place.)