It is a jarring story in and of itself, made more acute by Mark's unusual amount of detail. In most cases, Mark gives the least detail of the four Gospels, but on this one event he gives more than the others.
Scholars tend to agree that Mark was written for Christians under persecution. Perhaps the account of a righteous and faithful witness, jailed and murdered by the powers-that-be, was important for them.
The powerful are portrayed in all their earthly glory. They drink and party at public expense, they seduce, they place themselves above written and moral law, they make expansive promises and use positions of influence to manipulate for petty, personal agendas. There's nothing the righteous can do when the rich and powerful decided to throw their weight around.
Yet Mark is clear that John is "righteous and holy" - even those who do him in know it.
Like the Christians for whom Mark wrote, we have to remember that earthly outcomes are not the measure of our worth in God's sight. That reality is affirmed in the evening Psalm,
But God will ransom my life; he will snatch me from the grasp of death.
Do not be envious when some become rich, or when the grandeur of their house increases;
For they will carry nothing away at their death, nor will their grandeur follow them.
Though they thought highly of themselves while they lived, and were praised for their success,
They shall join the company of their forebears, who will never see the light again.
Those who are honored, but have no understanding, are like the beasts that perish.