Christians were opposed to the practice; in his autobiography St. Augustine recounts his experiences at gladiatorial games and his reflections upon their effect on the individual psyche, and how this indulgence was counter to Christian virtues.

Furthermore, I cannot find the original source in which he said it, but 3rd century author Tertullian wrote in detail about the effects of gladiatorial games as attended by Christian populations, and regarded them negatively as well.

There is also the narrative of the martyrdom of Saint Telemachus, who is said to have attempted to intervene during a game, and was stoned by the crowd, or alternatively condemned the crowd for their endorsement of the games and was summarily executed by the gladiators by order of the prefect of Rome. While this story isn't strictly a historical account, its origin in the writings of Theodoret of Cyrus in the 5th century serves to illustrate the early church's stance on gladiatorial combat.

From another angle, Roland Auguet's book "Cruelty and civilization," describes how, during the decline of the games many Roman officials disliked the implementation of gladiatorial games as they were a tax on the imperial coffers that could sparsely be afforded in the wake of the Imperial Crisis of the Third Century.