स यदि पितरं वा मातरं वा भ्रातरं वा स्वसारं वाचार्यं वा ब्राह्मणं वा किंचिद्भृशमिव प्रत्याह धिक्त्वास्त्वित्येवैनमाहुः पितृहा वै त्वमसि मातृहा वै त्वमसि भ्रातृहा वै त्वमसि स्वसृहा वै त्वमस्याचार्यहा वै त्वमसि ब्राह्मणहा वै त्वमसीति ॥ ७.१५.२ ॥
sa yadi pitaraṃ vā mātaraṃ vā bhrātaraṃ vā svasāraṃ vācāryaṃ vā brāhmaṇaṃ vā kiṃcidbhṛśamiva pratyāha dhiktvāstvityevainamāhuḥ pitṛhā vai tvamasi mātṛhā vai tvamasi bhrātṛhā vai tvamasi svasṛhā vai tvamasyācāryahā vai tvamasi brāhmaṇahā vai tvamasīti || 7.15.2 ||
2. If a person speaks rudely to his father, mother, brother, sister, teacher, or to a brāhmin, people say to him: ‘Shame on you! You have murdered your father. You have murdered your mother. You have murdered your brother. You have murdered your sister. You have murdered your teacher. You have murdered a brāhmin’.
Saḥ yadi, if anyone; pitaram vā mataram vā, his father or mother; bhrātaram vā svasāram vā, or brother or sister; ācāryam vā brāhmaṇam vā, or teacher or a brāhmin; kiñcit, anything; bhṛśam iva, discourteously; pratyāha, answers; dhik tvā astu iti, shame on you; enam eva āhuḥ, [people] say to him; pitṛhā vai tvam asi, you have killed your father; mātṛhā vai tvam asi, you have killed your mother; bhrātṛhā vai tvam asi, you have killed your brother; svasṛhā vai tvam asi, you have killed your sister; ācāryahā vai tvam asi, you have killed your teacher; brāhmaṇahā vai tvam asi iti, you have killed a brāhmin.
According to Śaṅkara, using the informal tvam (you) instead of the more formal bhavān would be such an offense. Here the Upaniṣad means that you should treat everyone with respect. Being disrespectful to people is as good as murdering them.