The Haftara from Shmuel Alef, 11:14-12:22 is a good match for Parshat Korach for a few reasons.
One reason is that both the Torah and Haftara readings deal with leaders who defend themselves using almost the same words.
In Parsha Korach, After Datan and Aviram (two members of Korach’s revolt) attacked Moshe for not bringing them to the land of milk and honey, we read (Bamidbar 16:15): “Moshe was extremely distressed and said to God: ‘Do not turn to their offering; not a donkey did I sequester from any of them nor have I done ill to any of them.’”
Rashi quoting Midrash Tanchuma comments that Moshe states that even when he needed a donkey for transportation, he did not take one that belonged to B’nei Yisrael. Rather, he took his own.
In the Haftara (Shmuel Alef 12:3) as Shmuel crowns Saul as king and gets ready to step down as judge, Shmuel says: “Here I am; testify about me in the presence of God and in the presence of His anointed: Whose ox have I taken? Whose donkey have I taken? Whom have I robbed? Whom have I coerced? From whose hand have I taken redemption-money that I should avert my eyes from him? And I shall make restitution to you.”
Rashi elaborates that Shmuel asked if he had ever used anyone else’s ox to plow his field or whether he had ever taken a donkey other than his own as transportation.
If you pay careful attention you see that Moshe addresses God, while Shmuel speaks directly to the nation and asks for their trust.
In the story of Korach, the rebels are angry at Moshe and his brother Aharon yet taking a donkey is not one of their complaints and in the case of Shmuel, the elders (Ziknei Yisrael) ask for a king since Shmuel’s sons did not follow in his path yet they are not angry at him as it says in Shmuel Alef 8:5: “You have grown old and your sons did not follow in your ways, appoint for us a king to judge us like all of the nations.” What bad things did Shmuel’s sons do? The answer is in Shmuel Alef, 8:3: “They were swayed by profit; they took bribes and they perverted justice.”
Rav Yisachar Yaakovson asks why both Moshe and Shmuel got all defensive about stealing donkeys when that was not what they were being accused of. His answer is that every complaint needs to be analyzed psychologically- what was said outright as well as the intention behind what was said. Both Moshe and Shmuel felt that their credibility was being attacked as often leaders are suspected of stealing (even in their time) and unfortunately in many cases the suspicions are correct.
In Psalm 99 which we recite as part of the Kabbalat Shabbat service we say: “Moshe and Aharon were among his Kohanim and Shmuel among those who invoke His name- They called upon God and He answered them.” We see from here that all three were considered the true leaders from the tribe of Levi.
Another reason why Parshat Korach and this Haftara from Shmuel are read on the same day can be found in the genealogy of Divrei HaYamim Alef (Chronicles) 6:18-23, where Shmuel is listed as a descendent of Korach. Rav M. Hirsch teaches that from the seed of Korach, the rebel who rose up against God’s kingship descended Shmuel, the strongest fighter for guarding the holiness of God’s leadership.