"Oh, how foolish you are! How slow of heart to believe all that the prophets spoke! Was it not necessary that the Messiah should suffer these things and enter into his glory?"
Then beginning with Moses and all the prophets, he interpreted to them what referred to him in all the scriptures.
His teaching, of course, caused their hearts to burn within them, though it evidently didn't cause them to record his teaching in any detail. So we're left with the question, "Was it not necessary that the Messiah should suffer?"
"anything which of its nature cannot be otherwise"; neither Divine nor human nature, of itself, made Jesus' passion necessary.
"the necessity of compulsion", when some outside cause acts upon a thing; nothing can act upon the Divine nature, which is literally impassible, and in His human nature Jesus willingly chose to suffer.
" necessary from presupposing [an] end"; this is the necessity St. Thomas sees, "and this can be accepted in three ways":
on our part, the end being our salvation
on Jesus' part, the end being His glory (here St. Thomas quotes the above verse from Luke)
on God's part, who willed our salvation and Christ's glory through His passion
Now, St. Thomas goes on to argue that "speaking simply and absolutely, it was possible for God to deliver mankind otherwise than by the Passion of Christ." The Passion was necessary because it was the Father's will, but the Father's will needn't necessarily have been the Passion. It's a necessity that has its origin, as all things do, in the Father's will.
But I think we can also speak of the necessity of the Passion presupposing the end of Jesus' disciples following Him (following Jesus being, in turn, the means to the end of our salvation). Which brings me, at long last, to Christ the Pierced King.
That He is our king means we must do His will. But doing His will means suffering, sometimes martyrdom. In His love for His people, Jesus cannot will us to do what He Himself did not will Himself to do; that would be to treat us as merely useful objects for effecting His will, rather than as beloved subjects.
If Christ is to be King, then He must be pierced for those who belong to His kingdom. His passion and His kingship are inseparable. Without the former, He would be merely an unloving despot; without the latter, He would be merely an exemplar of virtue. With both, He is True God from True God.