I read an argument that said death was only bad for an organism who could understand and plan for their future. The suffering caused by death for humans was the "prevention of future goals," such as having a family, making money, finding purpose, etc. He argued that lower species had no understanding of their existence through time, and only ever thought of the future. For example, he suggested that if a fox was caught in a trap, the fox does not know "if I remain in the trap, I will die." Yet, the fox chews through their own leg to escape. He thinks it's more likely that the fox merely considered the pain of a lost limb to be superior to the pain and anxiety caused by lack of movement. When a cat meows to be fed, he isn't saying, "I want to be fed three hours from now." He is saying that he wants food NOW, and by now, he means as close to the present as possible. If a cow has to walk to a spot of grass to feed, the cow isn't concerned with getting grass fifteen minutes in the future, but simply wants grass now and is accomplishing the necessary activity to find pleasure in the moment.
Essentially, animals do not lose anything by dying. They may have future pleasures, but they don't expect or care about those future pleasures because they don't know such pleasures will exist, thus their will isn't being negated and they experience no sadness from dying aside from the physical pain of being killed.