while i am inclined to agree with you that they are not ethical, also consider that most of the fights end in a meal for the victor. whereas if you were to collect and curate those same specimens yourself you would be happy and proud, even though the death of that bug would not even result in the nourishment and continued life of another.
the only reason anyone gets upset by the bug fights is that the specimens in question are desirable or interesting or difficult to obtain/keep. if it were a video of "boring" bugs killing and eating each other nobody would care. in all reality you perform a similar experiment every time you feed cockroaches or crickets to a tarantula, scorpion, or centipede.
...and also every time you eat a hamburger.
some animals eat others. some get eaten. that's how the world works.