Landry was
absolutely designed as what tvtropes calls the "sacrificial lamb":
http://tvtropes.org/pmwiki/pmwiki.php/Main/SacrificialLamb
A character, that
should give you the impression she's a main character, only to die very early on, to give the viewer the feeling of "no one is safe". Of course, she was listed as a "guest star". But how many viewers actually know the names of the actors? In the pilot of Firefly, every main character got a big "introduction" scene,
as did the one guy who turned out to be a traitor. That was intended as a surprise, slightly undercut that all the regulars got a shot during the credits, but he did not. Landry clearly got a very obvious "introduction to her character" scene the previous episode and was treated as a regular during the (second) pilot, only to be off-ed in a surprise twist. I'm fine with that. The only thing noticeably is, they
already did that before, with Cpt. Georgiou (and the entire
Shenzhou, really).
Some viewers seem to be so stuck in the ol' concept of “main characters” and “ensemble casts”, where the only time a character dies was when the actor didn't renew their contract and had to be written out by the end of the season. There are no safe characters now (well, I guess, Michael Burnham is). The writers treat this like a novel, where characters have to go when their time's up, i.e. when it makes the most sense from a dramatic point of view.
You're absolutely wrong if you think Landry was killed to because "her time was up", or even that it made "the most
sense" dramatically killing her in this situation. She really, only, purposefully was killed to give viewers the feeling of "There are no safe characters now" (as you said it). But the specifics of her death didn't weren't important for the writers or made much sense. They could also have easily killed her during the final battle. But they wanted to give audiences a big "Oomph" during the
middle of the episode, to focus on that, and not have it drowned out by the spectacle. So, stupid misjudgement about monsters it was.