Both けもの and けだもの are written in kanji as 獣, but is the beast with 'だ' inside more brutal than the one without 'だ'? Or are they simply two variants of the same noun, thus they refer to exactly the same type of beasts? If they are the same word, why is 獣道 only read as けものみち but not けだものみち?
Apparently the reading comes from "毛の物", (as くだもの = 木【く】の物), so both mean "beast" as in "furry mammal" (although I'm sure it will stretch to cover those hairless cats).
けだもの has an additional meaning that けもの doesn't, when applied to people (strongly negative, like "he's a monster").
That might also be the reason that 獣道 uses the more neutral reading けもの (and also 獣偏 is けものへん).