First of all, the concept of 'human rights' that you are inserting into this is an anachronism. There were certain rights that you would have as a citizen, but there was no concept of certain rights extending to all human beings. Were there people who didn't like gladiator games and found them cruel? No doubt, and Cicero does mention this, but there was no such thing as a political movement to free gladiators, or slaves in general.

Secondly, gladiators generally didn't fight to the death. They were highly trained professionals, and training them was an investment. Wealthy owners who owned gladiator schools didn't get rich by getting their investments killed on purpose.

Fights to the death would involve either criminals or slaves. In the case of first, it's a form of capital punishment; Without getting too much into this, I find it hard to accept the argument that this is somehow more cruel than the types of capital punishment that are in effect today.

As for slaves, the Romans were a slave owning society, and slaves were treated much like property. They were not considered to have a right to life. I don't know if dying for someone's entertainment is better or worse than living a miserable existence as a slave, but I'm glad I won't ever have to find out.

There's a very similar question here