When a kid I saw a film with that plot, First Man Into Space.
It starred Marshall Thompson, who later played "Daktari" in TV show of that name. At the film's end Thompson asks the captured man/monster why he'd done what he did; the (dying?) monster replies
"I just had to be the first man in space!"
Rivalry between brothers leads to main story line. Navy Commander
Chuck Prescott(Marshall Thompson)has developed the Y12 aircraft to
test how far man can go up in the atmosphere.
His brother, Lt. Dan Prescott(Bill Edwards), seems to be the best test
pilot around and is chosen to go up in the Y12. Dan of course has a
problem with taking orders and is also an over confident daredevil.
On Dan's second flight, he hits over the 300 miles up comfort zone and
his craft passes through a meteor dust storm. Returning to earth, Dan
becomes a monster that resembles 200 pounds of bad asphalt. He also
has a demanding craving for blood, whether it be from farm animals or
fellow human beings.
Short runtime of an hour and seventeen minutes; black & white with
near stoic acting...typical of low budget sci-fi.
Rounding out the cast is Marla Landi, Robert Ayers and Carl Jaffe.
Noteworthy trivia: about two months after this film was released; the
Russians put the real first man in space. Scary as hell