Some months ago I watched Mission to Mars and in the first part of the movie you see that the astronauts are killed by a large vortex generated from the humanoid base. In the end you'll see that the aliens help humans to understand their origin, after the transmission of the pair of missing chromosomes code. Isn't this contradictory?
The reason for this is that the "aliens" were waiting for the correct response to their signal. If they got the wrong response, their response was the vortex. The reasoning for this was because if the responders could not understand the message and respond accordingly, then they would not be getting the rest of the information. It's a sort of a if you aren't ready for it, you aren't getting it situation. Towards the end of the movie, the astronauts (Gary Sinise and crew) then give the correct response and so are allowed to survive to receive the rest of the answer.