The movie starts with a flying saucer landing in an area that looks like a desert type area. Three space men get out and walk away from the ship. They look just like us and two are holding ray guns. It seems that they have come to Earth to execute the third man. There is a sudden comotion in a cave close by and in the confusion the man to be executed runs off. There are two plots to the movie. The first is the two looking for the one to be executed. The second plot is the one man trying to help the earth people in killing a giant crab like creature that has escaped from the cave. I remember that in the end he uses a power line to power up his ray gun so he can kill the creature.

Hi itsbubba. The first movie that popped into my head was Attack of the Crab Monsters. A very fun classic. BUT I realized that there weren't aliens in that one.

Although it is slightly different in that their original mission was not to execute their alien team member but to find a suitable habitat for their creatures, the rest of your description (ray guns, the guy running away & helping humans, the aliens trying to chase the guy down), convinces me you are looking for Teenagers from Outer Space. The creatures are closer to looking like lobsters rather than crabs. Like Crab Monster, this is also a fun classic.

Teenagers from Outer Space - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Teenagers_from_Outer_Space)

Teenagers from Outer Space (1959) - IMDb (http://www.imdb.com/title/tt0053337/)