अनुबन्धं क्षयं हिंसामनपेक्ष्य च पौरुषम्। मोहादारभ्यते कर्म यत्तत्तामसमुच्यते ॥

anubandhaṁ kṣhayaṁ hinsām anapekṣhya cha pauruṣham mohād ārabhyate karma yat tat tāmasam uchyate

The action undertaken from delusion, without regard to capacity and consequences—loss and injury to others—that is declared to be dark.

In plain words: A dark action is done out of confusion. The person does not think about whether they can do it, what will happen, or how it might hurt others.