Question

Why did Lord Krishna kidnap Rukmini to get married?

Answer

As per vedas getting married to a girl after kidnapping is considered devil/daemon (दानव) Vivaha (marriage) which isn't righteous (धर्मानुकूलन).

You have perfectly stated the correct thing. Shri Krishna indeed married Rukmani in a rākṣasa style:

bhagavān bhīṣmaka-sutāṁ rukmiṇīṁ rucirānanām
rākṣasena vidhānena upayema [SB - 10.52.18]

Meaning
King Parīkṣit said: The Supreme Lord married Rukmiṇī, the beautiful-faced daughter of Bhīṣmaka, in the Rākṣasa style — thus I have heard.

As per our scriptures marrying a girl by kidnapping her is considered to be a marriage of the demonic type (danav vivha / rakshasa vivha):

The forcible abduction of a maiden from her home, while she cries out and weeps, after (her kinsmen) have been slain or wounded and their houses broken open, is called the Rakshasa (demonic) rite. [Manu Smrt. - 3.33]

However, Shri Krishna, as a kshahtriya, did not break any law or dharma by such marriage. Because the scriptures allow danav vivah for the people of the ksahtriya (warrior) caste:

For Kshatriyas those before-mentioned two rites (pichasa and asura), the Gandharva and the Rakshasa, whether separate or mixed, are permitted by the sacred tradition. [Manu Smrt. - 3.26]

Now regarding the question why Shri Krishna kidnapped Rukmini, it is because She loved Him and She Herself sent Him message to do so:

śvo bhāvini tvam ajitodvahane vidarbhān
guptaḥ sametya pṛtanā-patibhiḥ
parītaḥ nirmathya caidya-magadhendra-balaṁ prasahya
māṁ rākṣasena vidhinodvaha vīrya-śulkām [SB 10.52.41]

Meaning
O unconquerable one, tomorrow when my marriage ceremony is about to begin, You should arrive unseen in Vidarbha and surround Yourself with the leaders of Your army. Then crush the forces of Caidya and Magadhendra and marry me in the Rākṣasa style, winning me with Your valor.

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