Regarding international law, just check on
http://en.wikipedia.org/wiki/Geneva_Convention_(1929)
That's 1929 law.
I would argue that mass murders are always against international law... but that aside, a large number of the deaths in the concentration camps were Soviet prisoners, including soviet JEWS... they worked, starved, died, and were killed just like anyone else.
Regarding Stalin's actions. Yes, what you say is true. But it wasn't because they were religious or Ukrainian. It was because those individuals posed a threat to his power. Religious leaders are LEADERS. Nationalists are LEADERS. Stalin's purpose was to cement his authority. And he tried to justify his actions with farce trials. The Nazis just LITERALLY rounded up people and killed them. They would present a list of names (or request said list) to local governors/leaders.... and they would expect all of those people to be shipped off to camps.
All of this is very well documented, and can be found easily on wikipedia. There are a lot of heroic tales of people helping commoners and civilians whose only crime was that they were Jewish. There are documented tales of leaders who were under Nazi occupation who tried to slow the trains down. Leaders who were basically in the position of give us the Jews or we will kill you and your leaders and replace them with Nazi officials.
No one can defend those actions. There is just no defense.