I didn't say either was right or wrong I just said that it was fair. The way you feel about a conflict doesn't change how either side fight. And your historical examples are only relevant because we were on the winning side so of course we fell it was the right thing to do. But during that time period the American public was very against getting involved with another war in Europe after WW1. FDR was looking for every way possible to convince the American public to support the Allies but during that time period we were very isolationist.
So to answer your question, No Sweden should not have sent weapons to the Germans in 1941. Not because the US sent weapons to the UK but because Sweden was neutral by their own policy.
I find it funny that you're using the UN Security Counsel as some sort of authority. Could you tell me who are the 5 permanent members of that security counsel? Those resolutions are only as legal as the they are enforceable. Honestly if North Korea wants to supply weapons to Russia and Russia accepts then who can stop them? No other country or entity has any authority of either. The best you can do is sanctions or war. But to give another example of how the UN has no power unless granted power; Everyone seems to forget that NATO, a defense alliance, attacked a sovereign European nation.
-- From Wikipedia.
So here we have NATO itself ignoring the UN Security Counsel.