After watching this, I realized that The Hidden
(1987) copied much of the premise from here - the bad alien on Earth pursued by the good alien. Both aliens can take possession of Earth entities. A nuclear physicist (John Agar) and his buddy (Robert Fuller) head to a desolate desert location to check out strange radiation readings. They find a new cave - uh, oh - and inside is... the titular character.
The bad alien is on a power trip; its plans are obvious (to rule! to take over!). With Agar under its control, the alien likes to express itself with a maniacal grin while zapping airplanes or people, its eyes luminous. It also behaves very amorously towards Agar's fiancee (Joyce Meadows), to the point that she calls him a caveman. Then it holds a meeting with military & political leaders, explaining its grand plan -- bwha-ha-ha-ha
Each alien has a distinctive voice; the bad alien, Gor, has a very arrogant, melodramatic voice. The good alien, Vol, has a very calm, rational voice. The ending is baffling in its abruptness; it seems like a happy ending, but I'd be interested to see how Agar's character explains his actions to all the leaders who witnessed him commit mass devastation. BoG's Score
: 5 out of 10