Contradiction with Matthew 5:4
This verse suggests mourning is blessed and comforted, whereas 2 Samuel 12:21 questions why David ceased mourning.
Matthew 5:4: Blessed [are] they that mourn: for they shall be comforted.
Contradiction with 1 Thessalonians 4:13
This verse encourages believers not to grieve without hope, contrasting with the implication in 2 Samuel 12:21 that grief was expected.
1 Thessalonians 4:13: But I would not have you to be ignorant, brethren, concerning them which are asleep, that ye sorrow not, even as others which have no hope.
Contradiction with Ecclesiastes 3:4
This verse speaks of a time to weep and a time to laugh, yet 2 Samuel 12:21 shows a lack of understanding for the change from grief to acceptance.
Ecclesiastes 3:4: A time to weep, and a time to laugh; a time to mourn, and a time to dance;
Paradox #1
The contradiction might be that the people around David are confused about his behavior. Before the child died, David was very upset and didn't eat. But after the child died, he stopped mourning and ate. This could be seen as inconsistent because people usually expect someone to mourn even more after a death, not less.