There is a paradox here. By right humans are the voidbringers and we stole Parshmen planet. So from a Radiant pov it's the Parshmen who are in the right. They are caught in a dilemma to choose between humans vs Parshmen.
It's not really easy to just choose millions of humans as you say and kill Parshmen who are just trying to get back their home planet. More of a moral and philosophical debate than you say...and there seems no resolution. So that's why the Radiants foresook their Oaths. So you kill Parshmen who in this case cannot protect themselves from humans?
And most importantly it was Gavilar and Ghost Bloods (humans ) who was trying to bring back Desolation (and Radiants) while Parshmen were trying to keep their god ( Odium) dead.
I don't believe their oaths are restricted only to humans and ignores everything else. Doesn't make sense in bigger picture.
As to setting aside being a Radiant, too many explanations possible. Old age, done enough etc. We can only speculate now. Have no idea on this. RAFO, I believe.