Let's shift the analogy here a little; let's say you're a cast member in one of the Aliens movies. You know how the little buggers work, and you just saw one of them implant a crewmember.
You know that person is dead. Toast. All that you can do at this point is protect the rest of the crew by eliminating the parasite before it can spread.
Which is more moral; throwing the crewmember out an airlock, or letting the parasite hatch and probably kill you and the rest of the crew... leaving the crewmember dead anyway?