The Hidden
(1987)

Sgt. Tom Beck (Michael Nouri) is shot by the "evil alien", and is taken into a hospital.

After the "evil alien" takes over the body of the presidential candidate, FBI man and "good alien" Gallagher (Kyle MacLachlan) attacks him with a flame thrower, thus forcing the alien creature to get out. Gallagher is then able to kill the alien with his special weapon.

Sgt. Beck dies in the hospital. Gallagher takes over his body, and thus becomes part of the intact family that he had admired before.

Thanks Tilman!