I've always liked the cabal. More of the alien empire invading, hive are the only pure dark group. Vex have multiple different theories(created from traveller, darkness infected them, were created by the "void", etc) Fallen were probably the first race that the traveller gave powers to. Being a savage race the traveller left than and the fallen, now maybe a little dark but not really, seeks to have revenge on the race that took the traveller from them, Us. Also only half of the vex are dark, on half atheon Which is more nuetral to light or dark, and the sol divisive who are the black garden vex who are dark