The Atomic Submarine

A submarine is destroyed near the North Pole by a mysterious undersea light. The loss of this and several other ships in the Arctic alarms the world. Governments temporarily close the polar route and convene an emergency meeting at The Pentagon. Present is Commander Dan Wendover (Dick Foran), the Captain of the atomic submarine Tigershark, and Nobel Prize winning scientist Sir Ian Hunt (Tom Conway). The United States Secretary of Defense (Jack Mulhall) leads the meeting; he explains all that is known about the Arctic disasters and then describes the high-tech capabilities of Tigershark. These include a special hull and a mini-sub (Lungfish) that can be stored inside the submarine. The Secretary finishes by telling Wendover that he is to take Hunt, the Tigershark, and her crew to resolve the ship sinkings and, if possible, eliminate their cause.
Lieutenant Commander Richard "Reef" Holloway (Arthur Franz), Tigershark's Executive Officer, learns that his bunk mate is to be Dr. Carl Neilson Jr. (Brett Halsey), a pacifistic scientist he dislikes. A montage then follows, spotlighting the day-to-day life aboard Tigershark, which eventually discovers the cause of the disasters: an underwater saucer-shaped craft with a strange sole light atop and in the center of the saucer's upper dome. One of the Tigershark's scientists, Dr. Clifford Kent (Victor Varconi), briefly shows a photo of an Unidentified Flying Object, pointing out its similarity to this underwater UFO. The submariners began to realize their quarry is an extraterrestrial lifeform. The crew nickname the saucer "Cyclops" because of its sole light.
Commander Wendover orders the submarine's most powerful torpedoes fired. They reach the saucer but do not explode, being stopped by a gel-like extrusion coming from within the UFO. The captain knows that he must stop the ship sinkings, so he orders Tigershark to ram the alien saucer. The submarine's bow tip breaks through its lower side and becomes trapped.
Dr. Neilson pilots Lungfish, taking Lt. Commander Holloway and a small party to board the UFO. While Dr. Neilson waits on the mini-sub, Holloway has the boarding party cut free the bow with blow-torches. Meanwhile, he explores the saucer's dark hallways and receives his first telepathic message from its sole occupant, an octopus-like creature with a single large eye for a head. The alien proves its hostile intent by remotely killing all the boarding party except Holloway. The creature announces that it plans on bringing several human specimens back to its home planet for further study. The aliens' plan is to modify themselves, based on what they learn about humans, and once finished, they will come back and take over the Earth.
Holloway attacks the alien by firing a Very pistol into its single eye, temporarily blinding it. He then races back to Lungfish and returns to the Tigershark with Dr. Neilson. When Dr. Neilson asks about the remainder of their boarding party, Holloway says, "Fortunes of war".
Despite the large hole in its underside, the flying saucer sails to the North Pole to recharge its energy in preparation for take-off. Holloway tells Commander Wendover, "Captain, if that thing ever gets back to where it came from, the Earth and everyone on it is doomed".
The submariners hold an emergency meeting of Tigershark's on-board scientists, and they develop a plan to adapt a torpedo's guidance system to an ICBM, converting it into an improvised ground-to-air missile. When the saucer rises from the ocean, Tigershark fires the missile, destroying the UFO. Holloway and the young Neilson are reconciled, with the latter realizing that his pacifism was no match for a hostile alien.

Question: How does the alien prove it has hostile intent?
Answer: killing all the boarding party except Holloway.