Krishna kidnapped Rukmani because Krishna was a warlord and not a God or divinity of any sort as per the Vedic tradition.
Kidnapping a girl and marrying is not regarded as an approved means of marriage in the Vedic civilization i.e. among Aryans (more to the this below). Even the Manusmṛti (see 3.41) condemns it by saying that children born out of such marriage are nāstika, cruel and are dharmadvesi (i.e. act against Dharma).
Kidnapping and marrying girl was trend well established in the tribal and warlord communities. The remains of this tradition can still be observed in the traditional marriages in North India (e.g. Baraat), wherein bridegroom travels on the horse, brandishing a sword in his hand, to "bring" the bride.
Traditionally and still prevalent in many parts of UP, Rajasthan, Punjab and Gujarat, only men are the participants in such procession. Such an abhorrent practice is still very popular in countries dominated with the tribal culture, for example, Kyrgyzstan (see this ).
In contrast to such nonsensical "religious" acts, in the Vedic civilization, son and daughter were considered equal and the father with a greater number of daughters has been praised (RV 6.75.5).
बह्वीनां पिता बहुरस्य पुत्रश्चिश्चा कृणोति समनावगत्य ।
इषुधिः सङ्काः पृतनाश्च सर्वाः पृष्ठे निनद्धो जयति प्रसूतः ॥५॥
Furthermore, the prayer for fame has been made in Atharva Veda for a qualified daughter (AV 10.3.20). Daughter has been considered as a source of pride (RV 10.159.3).
मम पुत्राः शत्रुहणोऽथो मे दुहिता विराट् ।
उताहमस्मि संजया पत्यौ मे श्लोक उत्तमः ॥३॥
The same has been expressed in Āpastamba Dharmasūtra (1.11.31.8 and 2.6.14.4).
In Śatapatha Brāhmaṇa prayers are being made for an intelligent daughter (SB 14.9.4.16).
अथ य इच्छेत् दुहिता मे पण्डिता जायेत सर्वमायुरियादिति तिलौदनं पाचयित्वा सर्पिष्मन्तमश्नीयातामीश्वरौ जनयितवै
In the Vedic times, the bridegroom was selected by the father (Vājasaneyi Saṃhitā 12.62). Furthermore, it is mandated that the daughter should accept a man who follows the regulations of the Scriptures, erudite and embodies noble qualities, free from duplicity and desires only one woman (Vājasaneyi Saṃhitā 12.62).
असुन्वन्तम् अयजमानम् इच्छ स्तेनस्येत्याम् अन्व् इहि तस्करस्य ।
अन्यम् अस्मद् इच्छ सा त इत्या नमो देवि निर्ऋते तुभ्यम् अस्तु ॥
The Vedas mandate that the daughter should be protected and should not be pained (AV 8.6.25).
अप्रजास्त्वं मार्तवत्समाद्रोदमघमावयम् ।
वृक्षादिव स्रजं कृत्वाप्रिये प्रति मुञ्च तत्॥२६॥ {१६}
The concept of marriage by abduction is not present in the Samhitas or the Brahmanas. This is another instance as to how warlord and tribes became “Gods” in our times and still lay claim to the Vedic heritage!
It is the need of our times that the respect and sanctity of women should be restored and discard lame excuses to glorify such immoral transgressions of the Krishna or that matter any mythological warlord.
The Vedas never condone such wrongdoing not even for warlords!