Who says that the lion isn’t just needlessly cruel? You can only assume that it lacks the ability to make moral decisions, but then again humans kill children all the time for incomprehensible reasons. How are these lions and humans different except your perception that one has moral agency and the other doesn’t based on absolutely no empirical evidence except your belief in your own superiority.
Again this entire thing hinges on the notion that animals lack rationality, but the evidence increasingly dies not support that. But that’s neither here nor there, the nature of a lion is different to the nature of a human. And even then, we still do what you have described all the time. It may be wrong but we do it, and we will still do it a thousand years from now if we are still around. Now the argument is not because “it is part of your nature, you’re allowed to do it”, the argument is that all animals have a right to kill other animals in order to defend or feed themselves.
The other part of the argument is that unless you think for example another omnivore like a chimp (which fyi we have absolutely no reason to believe are any less rational than us ) is immoral if they decide to eat meat having plant based foods available then you shouldn’t think that about human beings.
If you want to improve animal welfare, you need to start believing that all animals, including human animals are equal. While society continues to believe humans are superior in any way to animals we will not be able to create a world in which all species are equally respected.