The word “passion” derives from the Latin term passio, which means “suffering, enduring.” The suffering He underwent was inhumane and horrifying. It began with mocking, abandonment, beating, scourging and a fake, fixed trial, and ended with crucifixion. It should give anybody serious pause when contemplating it. This passage from our study aid Jesus Christ: The Real Story explains a bit of what it was like:

In a crucifixion, the Romans typically used iron nails, five to seven inches long and about three eighths of an inch square, driven into the victim’s wrists and feet to fasten him to the wooden members. The Bible says nails were driven through Jesus’ hands, but in the language of the day the wrist was considered part of the hand. Nails were driven into the wrists, between the arm bones, because the hands themselves could not support the weight of the body. […]

The nails pounded through the wrists would have crushed the median nerve, the largest nerve going to the hand, causing indescribable pain. “The pain was absolutely unbearable,” says Dr. Metherell. “In fact, it was literally beyond words to describe; they had to invent a new word: excruciating. Literally, excruciating means ‘out of the cross.’

“Think of that: they needed to create a new word, because there was nothing in the language that could describe the intense anguish caused during the crucifixion” (Alexander Metherell, M.D., quoted by Lee Strobel, The Case for Christ, 1998, pp. 197-198). Nails driven through the feet would have brought similar pain.

We can’t know for sure whether Jesus was crucified on a simple stake or a cross with a crossbeam. Either way, being hung by His arms would have caused great stresses on His body. His arms would have been stretched several inches and both shoulders likely were dislocated.

The prophecy of Christ’s suffering in Psalm 22:14 refers to His tortured condition: “I am poured out like water, and all My bones are out of joint; My heart is like wax; it has melted within me.” (Did Jesus Really Die and Live Again?)

Why did He have to die? Because God requires blood to be shed for the forgiveness of sins (Hebrews 9:22). Until Jesus Christ came, God’s people sacrificed animals as sin offerings. Jesus Christ’s sacrifice eliminated the need for that by providing one sacrifice for every sin ever committed by anyone, if they choose to accept it. This passage from our study aid God’s Holy Day Plan: The Promise of Hope for All Mankind explains:

Christ’s sacrifice is the pivotal event in God’s plan to save humanity. Jesus foretold the fact that He would be “lifted up” in crucifixion so that “whoever believes in Him should not perish but have eternal life. “For God so loved the world that He gave His only begotten Son, that whoever believes in Him should not perish but have everlasting life” (John 3:14-16).

We see here that Jesus’ sacrifice was a supreme act of love for humanity. It is the most momentous step in God’s plan. The day on which this profound event, the crucifixion, transpired was the 14th day of the first month of God’s calendar, the very same day on which the Passover lambs were to be slain (Leviticus 23:5). Paul would later tell the congregation at Corinth that “Christ, our Passover, was sacrificed for us” (1 Corinthians 5:7). (Passover: Why Did Jesus Christ Have to Die?)

This begs the question, why did He have to be not just killed, but suffer so much? That is outside the scope of this Q&A, but you can find a bit more on that subject here and here.