Dallas is the captain of the Nostromo and might as well be the poster boy for awesome science fiction captain. He's brave, intelligent, and he's handsome in that late-70s manly kind of way. The actor that played him, Tom Skerritt, even got top billing. Ladies and gentlemen, we have a protagonist!
Record scratch. Alien isn't your typical science fiction movie, and there's more (or less) to this character than the sum of his individual traits.
Captain, Oh, My Captain
Let's take a moment to compare Dallas to some other famous starship captains. Like Kirk, he's brave and ready to act. Like Picard, he is intelligent and listens to classical music—specifically, Mozart's Eine kleine Nachtmusik or "a little night music." And like both captains, he is willing to take responsibility for his crew and ship.
But this isn't a feel-good adventure in post-scarcity liberal tolerance; this is Alien. In this context, Dallas's traits have some terminal downsides.
For example, when the facehugger latches itself onto Kane, Dallas attempts to get him aboard the ship. The hallmark of all good captains is that they never leave a man behind, right? Well, maybe they should—as this confrontation with Ripley, who is following quarantine laws, shows:
Dallas: Ripley, this is an order. You open that hatch right now, do you hear me?
Dallas: Ripley, this is an order! Do you hear me?
Ripley: Yes, I read you. The answer is negative. (Alien)
Dallas may be a brave, loyal captain, but he's not exactly thinking rationally here. When he disregards Ripley's warnings, he acts in the heat of the moment, thinking only about Kane and not the further-reaching consequences—which turn out to be pretty dire.
Even his later heroics can't make up for this lapse in judgment. Once the alien becomes a threat, Dallas devises the plan to force the alien out an airlock by forcing through the ventilation system and even does the courageous, captain-like thing by going in after the creature himself. But unlike Kirk versus the Gorn, it doesn't end too well for him.
Lesson learned: sometimes you do have to leave a man behind—especially when that man comes attached to prime, grade-A alien.
A Company Man
Dallas's other big mistake is putting his faith in the company. When company contract says to go to an unknown planet and explore an unknown signal from an unknown origin, Dallas says they go. When the company replaces Ash with Dallas's regular science officer and gives Ash complete control over all things related to the science division, Dallas accepts it. Plus, Dallas has access to Mother for most of the film and could easily have dug into Ash and his mission if he'd taken a few minutes to ponder why so many weird things kept happening.
His explanation for all this is simple: "It happens because that's what the Company wants to happen" and "Standard procedure is to do what the hell they tell you to do" (Alien). But this trust has deadly consequences—and it shows us the dark side of obedience.
(Pro tip: if you ever find yourself in a sci-fi movie, never trust any organization that calls itself "The Company." You can thank us later.)