According to the theory of karma, all our suffering and happiness is because of our past karma. If that is so, then why should someone, according to Bhagwan Krishna, raise hands against injustice and adharma?
The reason is that Jivas do not have the right to judge another Jiva. That right is reserved for Ishvara.
It is Ishvara who gives the fruits of karma. The fruit of action is from Him, this being the logical position.
Brahma Sutra Bhasya of Sri Sanakracharya III.ii.38
A Jiva only has the right and in fact the duty to serve a distressed person and fight against injustice and adharma. A Jiva has not been given the right to not serve another using the pretext that the person is suffering because of his past karma.
I abide in all beings as their inner-most soul. Disregarding My presence within them, men make a show of worshiping Me through images. If one disregards Me present in all as their soul and Lord but ignorantly offers worship only to images, such worship is as ineffective as a sacrificial offering made in ashes. A man who persecutes Me residing in others, who is proud and haughty, who looks upon God as the other – such a person will never attain to peace of mind. If a man disregards and persecutes fellow beings, but worships Me in images with numerous rituals and rich offerings, I am not at all pleased with him for proffering such worship. A man should, however, worship Me in images, side by side with discharging his duties, which include the love of all beings, until he actually realises My presence in himself and in all beings. As long as man is self-centred and makes an absolute distinction between himself and others (without recognising the unity of all in Me, the Inner Pervader), he will be subject to the great fear of Death (including every form of deprivation of self-interest). So, overcoming the separateness of a self-centred life, one should serve all beings with gifts, honour and love, recognising that such service is really being rendered to Me who reside in all beings as their innermost soul.
Srimad Bhagavata Purana III.29.21-27
You can have a legitimate question here. Can our helping a distressed person interfere with Ishvara's decision to allow the person to suffer? The answer is no. If Ishvara has really decided to allow a person to suffer then no attempt by any Jiva can reduce that person's suffering. Divine will overrides all effort by Jivas.
Brahma said, ‘No embodied being can counteract His will by austerity, meditation, Yogic power, intellectual skill, or worldly power. Neither with the help of others nor by oneself alone can one do so. Man takes on the body given to him by that mysterious Power for the very purpose of undergoing the experiences it subjects him to – birth, death, work, bereavement, grief, infatuation, fear, happiness, misery etc. We all offer our tribute of works as offerings to Him as cattle pulled by their nose strings do to their masters. The Veda consisting of sounds is the cable to which we all are tied with the smaller but stronger strings of the three Gunas of Prakriti and works springing from them. He controls and directs us with this rope, as their owners do their draught animals. O dear one! We are like a blind man, directed entirely by another with eyes. The Lord gives us each a body according to our eligibility through our nature and karma, and with that we undergo the sufferings and enjoyments He awards us.’
Srimad Bhagavata Purana, V.1.12-15