That is true. That's something I always liked about this movie when I was a kid. I always liked how brave and strong Lizzie was, but I never realized just how MUCH she was until I saw the two previous versions of The Bat in which her character is a complete craven coward. In fact I once did a report on the movies for school and I put that in the essay.

Most definitely, Cornelia van Gorder is the hero and protagonist of this picture, with Lizzie Allen a secondary hero as she is the one who rescues Miss Cornelia when she is trapped and suffocating in the sealed, hidden room. It's nice to see women in these positions, without dependence upon men, in a film from 1959.

Hmmmm, good point, I hadn't considered that, but I wouldn't exactly call him a hero. Actually if you look at it, the film doesn't really have a hero, except for maybe Warner, but Miss Van Gorder is thewhat is it, antagonist, protagonistwhatever, she's the main character and she's the one who finds out everything.

All related plot points strongly suggest Mark wanted to find his Uncle's stolen loot without anyone else getting in the way.
It's no accident, Mark's been unambiguously portrayed as a squeaky-clean guy, beyond reproach. Many seem to have missed the point he's the story's sole protagonist. However, in yet another unexpected plot twist, the author chose to kill-off the story's only hero early on.
Unlike all other characters competing to find the hidden loot, Mark's was the only having any genuinely redeeming altruistic reason for doing so.
Had the author permitted Mark to survive he'd unquestionably have returned the stolen loot to its rightful owner. Not just to do the right thing. But of equal if not greater importance, to clear the family's good name recently defiled by his nefarious Uncle's unbounded greed.

Novastar_6 - I was wondering the same thing when I watched this a couple of days ago. There is nothing to indicate that Mark wants to get the blueprints without anyone else knowing. Even if he does see the house as being his and therefore he can come and go as he wants, I would have thought that he would have announced himself out of courtesy to Cornelia seeing as she was the tenant. He seemed like a courteous kind of guy.
I'm hungry - you buy lunch!

The house was his by this time Mark Fleming was his uncle's heir. I always figured he sneaked into the house that evening, even though his visit was expected, because Mark didn't want anyone else to know about the hidden closet behind the grandfather clock, where the blueprints were located.
He didn't seem to be trying to sneak the blueprints out of the house, because the Bat killed him before Mark could really do anything more than open the closet and remove the blueprints. We really don't know what he would have done next.