'We can build up the air supply for ourselves later': Huh?
I've always loved IT! since I was a kid, and in general it's better than a lot of similarly low-budget sci-fi from that period. Still....
Okay, they decide to kill It! by the simple expediency of letting all the air out of the ship. And after Marshall Thompson comes up with this brainstorm, any questions about how that would affect the crew -- who, one presumes, would find air a useful commodity during the remainder of their four-month trip back to Earth -- are waved away by Dabbs Greer with the offhand comment, "We can build up the air supply for ourselves later." "Right!" shouts Marshall.
"RIGHT!"??? Umm, just how do they plan to "build up" an air supply? Stand around exhaling? (Yes, I know, that's not "air" anyway.) Their spacesuit air would last a few hours at best, covering the entire ship. The whole thing is ridiculous.
Of course, deciding to kill themselves to save the Earth from It! might have been one way to eliminate this plot hole, but totally unacceptable to the audience. A better way would simply have been to trap the creature on one deck, let the air out there, and either suffocate him or let the sudden depressurization shoot him out into space (a la IT's semi-rip-off, ALIEN). But as it stands, this ending makes no sense at all. Even as a kid, this annoyed me!