One killing all other living creatures in this world does not commit a sin so heinous as that of killing a single Brahmana. The great Rishis have said that Brahmanicide is a heinous sin.
यस्तु सर्वमिदं हन्याद् ब्राह्मणं च न तत्समम्। ब्रह्मवध्या महान् दोष इत्याहुः परमर्षयः॥