Here is the thinking that I use to guide my judgments in these matters:
First, I am strict with myself about distinguishing between emotion and reason. I welcome emotion in my life; I've been indulging myself in all kinds of emotional upwellings in the course of watching those Ken Burns videos on the national parks. But when it comes to judgment, I put emotion aside. So, the image of a bunny rabbit being killed and eaten by a slimy serpent, isn't something to which I would immediately object on emotional grounds. (Although I still remember reading a news story a few years ago about these teenagers who, for their personal amusement, grilled live kittens. That one, I admit, broke my discipline.)
Avoiding emotional decisions is an important prerequisite to good judgment, but the real key is clear reasoning. My thinking here goes something like this: Events themselves have no ethical weight, ethics being a creation of humanity. If, in the course of wild life or environmental occurrences, animals capable of suffering should be subjected to a great deal of it and then perish, that is simply the force of nature on the world in which we live. We might judge these occurrences regrettable, if they bring about the demise of exquisite species or the collapse of whole ecosystems, or if they contribute to hardship for the human species and our civilization, but such judgments are contextual only and do not address the physical occurrences themselves--i.e., the proverbial bunny being crushed and eaten.
However, we human beings have an incredible facility for rational decision-making, which, almost by definition (although the premise took much of our history to be consciously recognized and established), makes us culpable for our action in a way that the action of lesser animals and environmental processes is not. Therefore, immediately, the question of motivation arises. Why is a given act committed? "Why" is a fascinating question; we hold it highest in regard among "who, what, when, where, why, and how," probably because it is such a uniquely rational concept--for there is no "why" in nature. There is only "what," and "how," and so forth. Why only applies when a conscious decision has been made. It is our question, the question of the most sapient of sentient beings.
When a controversial action has occurred, such as what you showed us in this video, the first thing you have to do is set aside your emotions--what we call our "personal opinion." The second thing to do is ask why the person(s) who committed the action, committed it. Then, to determine whether a wrongdoing has occurred, we compare the action and its motivation with the contents of our ethics. (Or, among many, the consultation with ethics is supplanted by a consultation with morals. But I afford no validity to moral systems; I deem them invalid.) It is our ethics which determine, personally, whether a wrongdoing has occurred and, if so, how serious it is.
The next question is to search for mitigating or aggravating factors. If we determine a wrongdoing to have occurred, are there mitigating factors which lessen its severity? Was the person responsible for this act mentally ill at the time? Likewise, what about aggravating factors? Could the rabbit have been killed more humanely and then provided to the snake, while still providing the same food utility to the snake?
As more questions are explored and answered, a better judgment can be made as to the ethical character of the action, which then suggests our appropriate response. That's how I work through it. In some ways, this is similar to how our legal system works. In other ways, it differs. The religious among us would point out that, to them, some actions are inherently always wrong or always right--which of course contributes to the rigidity of religion, but that's another story.
I'm not sure whether or not the video you showed us is an example of animal cruelty or not. I am assuming the snake went on to eat that rabbit. If not, then, yes, unless there was some kind of meaningful science project underway, I would consider that to be animal cruelty. (And, even if there was a scientific purpose, I would want to know why it was necessary to feed the rabbit to the snake without sedating it.) But, if that rabbit was snake food, then, to the extent it is ethical for people to keep carnivorous pets at all, I would be hesitant to declare animal cruelty.
The most telling aspect of this is that the person filmed it and put it on the Internet for all to see. Why did they do that? Answer that question, and I suspect I'll be able to tell you whether or not I deem animal cruelty to have occurred.
One last comment. A question, actually, a question for all of you: When you hear the cries of a suffering animal (or person) and object to their suffering, is your objection raised against their plight, or your own? I harbor no small contempt for people who object to unsightliness and injustice elsewhere in the world simply because it makes them uncomfortable to watch. Yet, despite what most people will tell you, I suspect quite strongly that more objections are made in people's own interests than in the interests of the oppressed.