If Indy knew opening the Ark would kill whoever looks at what comes out, why was he so adamant about stopping the Nazis from getting it? He knew they were planning on opening it up in the island to see what was in it. I guess the easy answer is he wanted to save Marion who probably would have looked and died as well, so was the whole threatening to blow the thing up a farce so he could be captured with her and warn her? Just seemed a strange way to go about it. And if he knew how deadly looking at whatever comes out of it was, why not warn Marion, Sallah, etc earlier in the movie. As in "if that thing is ever opened for some reason, never look at what comes out".
And the broader question is was the Ark supposed to be a metaphor for nuclear weapons? Common knowledge that German and American scientists were in a race to first develop a nuke and end the war, and the Americans ended up winning Lost Ark Slayer
. Just the whole bit at the beginning with Marcus saying whatever army has the Ark would be invincible sounded like he could have been talking about nukes as well.