Indiana Jones and the Kingdom of the Crystal Skull

Source: http://www.imdb.com/media/rm56923648/tt0367882

I have just watched Indiana Jones and the Kingdom of the Crystal Skull yesterday night.

I am a fan of the Indiana Jones series, since I watched them in RTM2
when I was a kid, and played the LucasArts Indiana Jones' game as well.

However, I would rate this movie a 5/10.

I find that the plot of the story doesn't make sense.

The old man (friend of Indy whom became mad) was said to have been to the temple where there are the crystal skulls.

He supposedly reached inside and saw the alien there, but then managed to return back to the place where he found the skull originally.

However, towards the end, when the Ukrainian girl placed the skull back to the skeleton, she was instead incinerated by the alien.

So, this brings to mind, why didn't the old man replace the skull back to the skeleton when he reached the temple? If he did, why wasn't he killed by the alien?

All in all, the movie's plot had too many loopholes that it made the film seems childish.