Is it war crime to shoot down red cross marked plane?

So I was minding my own business then someone did this!

what was the international law in ww2 regarding red cross marked aircraft

Technically and morally YES....but, unfortunately, it is not always straight forward because, technically, if a aircraft/vehicle/ship that is carrying a Red Cross/Red Crescent emblem is spotted carrying out an illegal/hostile action, for example an aircraft drops bombs, paratroopers or illegal cargo such as weapons, it can be legally targeted.

Although the main rules regarding the treatment of the civilian population, POWs, injured combatants (included those being treated/moved under the auspice of the Red Cross/Red Crescent) and any mode of transport being used in such activities and marked with the Red Cross/Red Crescent emblem OR bearing a white flag cane into force with the creation of the normally recognised/quoted Geneva Convention Act of 1949 following atrocities carried out in WW2 there has, in fact, been a number of earlier conventions that dealt with the treatment of injured combatants - the first as long ago as 1864. This act and subsequent ones were subject to periodic updates but essentially remained as separate acts until the creation of the 1949 act which brought them all together.

See HERE, HERE and HERE for some detailed info.

If you read the third linked document you will see that some modes of transport that may be used as medical transports but do not normally carry a red cross emblem are also deemed to be covered under the convention, for example lifeboats belonging to a hospital ship or a national organisation such as the UK's RNLI (Royal National Lifeboat Institution).

I see, so having a red cross really doesn't mean alot on the battlefield

Exactly. It means something, just not a lot.

I decided long ago, the US and Great Britain are the only two that actually follow the Geneva Convention. Most everyone else consider the Red Cross to be a dandy target to aim at, whether it's on a plane, vehicle, or medic.

Of course, the ones who violate it the most are the ones that scream the fastest and loudest if there is even the appearance of the US or GB possibly violating some minor portion of the Convention, and are totally silent when their friends are shown, with photographic and witness evidence, to actually have violated most of it.

Just an opinion, based on a ton of "war stories". I've never seen it with my own eye, buuuut...

Pat☺