He was trying to paint an accurate portrayal of what the Roman Legion did to people. They didn't just put him up on the cross. The message was to love one another. That's what he told his disciples in flashbacks while he was suffering. The contrast there is what made it effective.
What was truly important about Christ's crucifixion was what the Bible points to in the event, not what Mel Gibson thought should be the emphasis given to his atoning sacrifice. Jesus was crucified along with many others. His manner of death, then, was not special; in fact, it was relatively common. God in His word draws attention instead to who was dying on a cross - Jesus, the perfect God-Man - and the spiritual and redemptive labor under which he suffered in taking our sin upon himself, bearing the wrath of God, and being separated from God the Father for a time.
Gibson's emphasis, then, on the physical suffering of Christ reveals both how little he understood of who Jesus was and what he was doing spiritually by sacrificing himself on a cross for our sakes.
In any case, none of this does anything to make watching horror movies okay. They are evil and watching them is sin - as the Bible makes crystal clear.