I prefer peace via neutrality & "splendid isolation", however, there are some scenarios that just cant avoid a nation standing idly by while another descends into chaos. A current example is the Turkey-Syria border crisis with "ISIS" besieging the Kurds in Kobani. The Turks are allowing civilians to enter their territory, but they could do more.
They should send humanitarian aid (food/medicine), weapons, vehicles, & airstrikes becuase if that town falls, "ISIS" will control most of the Syria-Turkey border & could send terrorists into their country, or worse, recruit radical young Turks amongst their ranks. Many Turks have already joined "ISIS", so for Turkey, the Kobani siege is a matter of national security.
I think it's morally justified for a nation to intervene in the domestic affairs of another nation if the people of that nation are being oppressed and mistreated. I think any time a nation has political regimes that abuse both their power and their people, it is then justified for another country to intervene and address the situation.
The world needs security, therefore it is morally justified for one nation to intervene in the domestic affairs of another. If other countries threaten the safety of ours or another world nation, then it can impact all of us. Our nation is obligated to protect our citizens, but also other countries that are in alliance with ours.
It is impossible to clump all domestic affairs into one neat box and say other countries should stay out of it. If this was the case people would have to turn their heads to Hitler-like antics. For the most part, it is safe to say that other nations should stay out of the domestic affairs of other nations. However, there will always be situations where it is necessary.
It is not morally justifed for one nation to intervene in the domestic affairs of another because I think we need to allow each nation to make his or her own decisions. Sometimes people do get put into power with no morals and they take advantage of the situation I would think these cases have to be handeled differently.