Man to Man in Araman!!!!!

[Kuruharan]

I'd still say that Finarfin had more moral fiber (and certainly less mule-headed stubbornness) than his brothers, even if it did take the Doom of Mandos to make him come to his senses. The point is that at least he came to his senses, which is more than can be said for Feanor or Fingolfin.

[Nilpaurion Felagund]

If I appear stubborn in my defence of Finarfin's action, forgive me. Like how you forgive someone for helping his grandfather.

After the Kinslaying, why didn't Finarfin - and Fingolfin - not leave the host?

They probably had the same hope as the Valar...

Now [the Valar] watched and waited, for they did not yet believe that Fëanor could hold the host of the Noldor to his will.

(The Silmarillion 9 - emphasis mine)

The Noldor was dismayed at the death of the Trees, and of their king, so they would have followed anyone who could explain what happened, and what they should do.

Of course, it had to be hot-headed Fëanor who had to do the explaining for them.

Now,

[Fingolfin] marched against his wisdom...because he would not be sundered from his people that were eager to go, nor leave them to the rash counsels of Fëanor...with Fingolfin went Finarfin also, and for like reasons...

(The Silmarillion 9)

In the Kinslaying, the only ones who participated are those who are loyal to Fëanor and those who didn't understand how the fight began. Thus the hope still lived that maybe this people will cool their heads - except Fëanor's host, probably -, turn back, and pretend like it's all a dream - except for a punishment or two for those who joined the massacre.

The main point of the opposition - Maédhros, that's you. - is that Finarfin only turned because of the Doom of Mandos. Scared, perhaps, or some other reason that still degrades personal morality. But I think the reason that Finarfin turned back only after the pronouncement of the Doom is that he realised that there will be no cooling of heads; or if their heads do cool, it would be too late.

...at the rear went Finarfin...and many of the noblest and the wisest of the Noldor; and often they looked behind them to see their fair city...

(The Silmarillion 9)

He still had a following. He still had cool-headed people who look up to him, who follow his lead. Would he move on, knowing the peril his people would face - be the Curse true or not?

He turned, because no-one else would stop and think of the consequence of their actions, except his people.