Contradiction with Exodus 21:16
Exodus specifies the punishment for kidnapping, which contradicts David's acceptance of Mephibosheth, who was not returned to Saul's family but rather integrated into David's household.
Exodus 21:16: And he that stealeth a man, and selleth him, or if he be found in his hand, he shall surely be put to death.
Contradiction with Deuteronomy 23:6
This verse calls for not seeking the peace or prosperity of certain people, contrasting with David's peace-offering to Mephibosheth in 2 Samuel 9:6.
Deuteronomy 23:6: Thou shalt not seek their peace nor their prosperity all thy days for ever. [prosperity: Heb. good]
Contradiction with Proverbs 24:17
Advises not to rejoice when your enemy falls, while David proactively seeks out and honors Mephibosheth from Saul's lineage, who was once his adversary.
Proverbs 24:17: Rejoice not when thine enemy falleth, and let not thine heart be glad when he stumbleth:
Contradiction with Matthew 5:44
Advocates for loving your enemies, which could be seen as different from simply making gestures to the house of Saul, as David did which could be interpreted as diplomatic rather than genuine love.
Matthew 5:44: But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you;