In Morte d'Arthur, how does the description of Sir Lucan's death contrast with the speech in which Arthur bemoans his passing?
Sir Lucan's death is described as a matter of fact; indeed, Malory is unimpassioned in his description of how the brave knight dies.
...and Sir Lucan fell in a swoon with the lift, that the part of his guts fell out of his body, and therewith the noble knight's heart brast.
In fact, Book 21 Chapter 4 tells us that, although Sir Lucan was 'grievously wounded in many places,' he continues to do the bidding of his king. Courageously, he makes his way to the battlefield to ascertain the source of all the commotion. To his grief, he discovers that there are robbers pilfering the jewels and precious ornaments from off the bodies of dead knights. Sir Lucan's only concern is for the safety of his king. To that end, Malory's terse description of the knight's death highlights Sir Lucan's stoicism and extreme courage.
We imagine that if Sir Lucan could describe his own death, it would be equally matter of fact and unsentimental. In contrast, King Arthur's speech is emotional and full of grateful lamentation about the fidelity and sacrificial spirit of his knight.
Alas, said the king, this is to me a full heavy sight, to see this noble duke so die for my sake, for he would have holpen me, that had more need of help than I. Alas, he would not complain him, his heart was so set to help me: now Jesu have mercy upon his soul!
Arthur's speech provides us a fuller glimpse of Sir Lucan's character; at no time do we read of Sir Lucan complaining or laboring in pain despite his grievous injuries. Arthur's brief eulogy describes the true courage and warrior ethos of a dignified soldier who will serve his king to his last breath.
I'm not sure "contrast" is an accurate word to use to compare Sir Lucan's death with Arthur's speech bemoaning his loss. Arthur points out an irony, if that's what you mean. Lucan is already mortally wounded when he tries to help Arthur to his feet. When Arthur swoons, Lucan, "in the lifting," swoons himself and "part of his guts fell out of his body,..." Then his "heart burst." When Arthur wakes up, he sees Lucan "foaming at the mouth and part of his guts lay at his feet." Arthur points out the irony that Lucan tried to help Arthur, when he was really in more need of help himself. Lucan's heart was dedicated to helping Arthur, which led to Lucan's own heart bursting from the effort. Arthur sees Lucan's act as noble, which presumably it was. I don't see any real contrast.