See this question in a video!
Our Friend The Expendable Crewman, Part 1

A: It depends. Were you the one who played the Sonic Sledgehammer, or was it an opponent? The Expendable Crewman’s power only triggers when someone else is taking or eliminating one of your Keepers. If you’re the one choosing a Keeper to get rid of (even just from a Keeper Limit) you’re not required to get rid of him.

Remember, it’s the person playing the Sonic Sledgehammer who gets to decide what Keeper everyone discards, including themselves. So if they are the one with the Expendable Crewman, they’re not required to discard that Keeper; they can get rid of whatever they want. However, if it’s someone else who has the hapless red-shirt, they have to choose for that player to discard the Expendable Crewman, even if there’s something else they’d rather that player lose.

To sum up, thematically: Your Expendable Crewman is only reckless when you’re under attack, otherwise, he’ll reliably go (or stay) where you want him to.