If looking at pandas brings humans happiness, then pandas are useful to humans. You are putting pandas in an inferior position to humans, because pandas are inconsensually being used by humans for to look at, therefore, humans are superior to pandas in this way. Humans have more powerful than pandas, which means even just one human can do much more than what all pandas can do together, therefore, it doesn't make sense to kill 1 human (even at random) rather than the whole species of pandas.

Besides all this, I think thinking about this question in a utilitarian way ignores the much bigger question, and that is the question of the value of life.

Both options (killing a random human and killing all pandas) aren't better or worse than the other.

If life itself is valuable, then all life is equally valuable. There is no form of life that is more valuable than another form of life because life itself is valuable. You can't put a number on the value of life either. If you claim that multiple lives are more valuable than one single life, you're saying that the value of life is based on the number of living things, which is contradictory to saying that life itself is valuable. Therefore, both options in aren't better or worse than the other.

Of course, we also have to consider that life has no value in itself. But then it doesn't matter which life you choose to end, because no life is valuable in itself. Therefore, again, both options aren't better or worse than the other.

This proves that killing one random human is not better than killing all pandas, but it's not worse either.

Edit: Thank yoi for taking the time to read this. :)