It needs to be established here that they are attempting to splatter a material that is, essentially, medical waste, that needs to be isolated and safely disposed of and not, in the result the Pragmatic Woman probably envisions, reduced to a liquid and evenly distributed over every nearby surface. It doesn’t matter. The tentacle sprays photogenic ichor but is not very inconvenienced by the inexplicably convenient projectile weapon. As it thrashes about, it decapitates Professorial Type, hits Pragmatic Woman with the same force but does not kill her, and enlarges the rupture enough for the thing that was Hennessy to penetrate further into the room. “Fall back!” the Captain screams. “Fall back!” And as the surviving members of the crew do just that, we can step away from all the inevitabilities that follow, from the increasingly desperate efforts to stop the creature in its tracks to the moments of valor that allow one crew member to sacrifice his life for another to the plans that don’t work and really, everything you already know is going to happen because you have already seen it happen any number of times, enough to ask just why the alien life form that transformed Hennessy is acting this way, if its ultimate goal should be to preserve the disease vector long enough for all these intrepid people to bring it back to the home world of their species.
Really; its actions, my actions, make no rational sense.
You want a successful disease? Try Toxoplasmosis. It is spread by terrestrial cats and except when contracted by vulnerable outliers like pregnant women, its key survival strategy is to ensure future generations by instilling in its hosts an attraction toward cats. Rats with toxoplasmosis don’t run from cats, as they should if they prefer to avoid being tortured and eaten, but toward them, drawn by a tropism that they might experience as love and/or as a baffling, incomprehensible and terrifying impulse. Eaten, they spread the toxoplasmosis to the cats, who then spread it to any other rats they may encounter but fail to kill, who then seek out more cats. It’s essentially a device to make sure that cats are loved forever. Toxoplasmosis is endemic among human beings and may well be the explanation for crazy cat ladies and the internet.
That is the way a successful parasitic disease acts, by encouraging the behavior that ensures its own propagation.
Now ask yourself, again, just what I think I’m doing, by turning Hennessy into a big scary glistening monster that his fellow crewmembers have every reason to want to kill, especially when—I won’t recount the actual moment but will assure you that it occurs—Hennessy continues to rant as he pursues everybody, reminding them again and again that he intends to infect all of human civilization. After all, wouldn’t it be easier for the disease to spread if Hennessy had remained the same tolerable jackass he’d always been, and if it had therefore had the chance to spread peacefully among a crew that would never have any real reason to suspect that it was sick?
You are very close to the answer now.
Draw a curtain over subsequent events, and you now find the two remaining members of the crew, now in quarantine over Earth.
Yes, we have skipped the entire climax. You don’t need to dwell on that part. Again, you have encountered it before, and you have internalized it, and you could almost certainly recreate it yourself if you were of a mind to.
At this point, Hennessy is gone, as the last three survivors managed to lure him into the ship’s core, one of them making the ultimate sacrifice in the process. They incinerated him, even as he screamed in final confrontation of his fate, with all appropriate pathos. Because nobody else ever came down with symptoms as extreme or as upsetting as his, it is widely presumed that he never passed on the contagion to anyone else, and that humanity is safe. It is true that he never passed on the contagion to anyone else and is also true that humanity is safe, but it is not true that humanity has gone uninfected. In fact, I now look out at the universe from billions of eyes, perfectly happy to be here, and perfectly happy to go without much in the way of overt mind-control for as long as your race’s policies of space exploration spread my diaspora throughout the stars.
The Captain is dead, the Cold Doctor is gone, the Hot Girl is gone, and the Guy With a Sweetheart Back Home is gone.
The Pragmatic Woman, who you likely imagined to be the hero of the piece, is also gone, as she was the one who pragmatically gave her life to help the other two survivors lure what was left of Hennessy into the all-consuming core.
Those survivors, who we now find hand in hand as they look down on the civilization they have entirely infected, are the Panicky Male and the Weepy Woman, both of whom, I can assure you, had a character arc, showing hidden reserves of courage and strength as the battle to save the ship came down to the last few minutes. They’re in love now, mostly because it strikes me as fitting and just that they should be. It amounts to a happy ending, I suppose. For them, and for me.
However disappointing they would find it, their hidden reserves of courage of strength were not entirely self-generated, but were the result of a chemical assist from inside, arranged by me.
You see, although they were all infected before they left the planet, every last one of them, it was entirely necessary that Hennessy transform into an object of terror. I could have picked any one of them to show obvious symptoms, but I assessed his personality and theirs at the moment of contact and thought he would be the most advantageous.
The Professorial Type and the Cold Doctor had to go, because they were the ones most likely to figure out what I was doing. The Captain had to go because, once everything went down, he was the one most likely to realize that piloting the ship into a sun was likely the best course of action. This also happens to have been true of the Pragmatic Woman. The Psychotic Type had to go because he was the most suicidally reckless, and in the unlikely event the true explanation of events had occurred to him, he would have had no problem killing everybody. I would have liked to save the Hot Girl and the Forgettable Guy and the Guy With Somebody Who Loved Him Back Home, but their demises, staged at key moments of the violent journey from that isolation cell to the ship’s core, helped steer the rapidly shrinking crew toward the actions I wanted, the ones that would accomplish what I needed to accomplish. The only two I really wanted to keep alive were Panicky Male and Weepy Woman, who were between them the two most likely in the aftermath to say, “Thank God we lived; it’s over now,” without ever really contemplating the premise that maybe it wasn’t. They were the two worst decision-makers, and so they did exactly what I intended them to do all along, the one thing I ran riot in Hennessy to accomplish: forgot all other considerations, and not incidentally, called off the rest of their survey mission, in favor of a beeline back to where I wanted them to be.