In this line, Frankenstein is probably refering to the monster he has created. By using the word "being" he is refering to a creature, something that is alive. Therefore the word "aspect" is used do refer to "the appearance of someone or something", in other words, he is talking about the way this creature looks. Frankenstein's monster appears to be so horrible his own creator was unable to sleep, remeber that it was created using dead body parts.