Thanatos was described as hateful in Theogony and Alcestis. Later, more favourbale depictions might have been driven by the same impulse that made Greeks call the Erinyes "Eumenides", the Kindly Ones: they wanted to avert heir wrath by placating them with kind words.  

 By the way: could anyone delete the info stating that Thanatos was the son of Erebos? Hesiod explicitly states that the dark personifications of negative phenomena like Death, Doom, Blame and other ones were children of Night alone, and she had them with no male partner.