Horizontal line z-axis not printing correctly
You'll notice in the screen capture that there is an icon marked "supports." This is precisely the solution required. When selected, the software should construct a series of vertical pillars in the locations required to properly print the horizontal portion of the knife blade.

It's common for Thingiverse models to have a notation Supports (yes/no) to indicate that this feature should be activated. Depending on the capability of your printer, some angles may require supports that are not required on someone else's printer model. There may also be an adjustment for angle reference to cease supports or to require. My printer will handle up to a 50° from vertical without support. Beyond that, more filament is used, but the result is superior to printing without it, as you've discovered.

Note also that support is typically categorized as "from the bed only" or "everywhere." The former prevents support from being generated between vertically separated parts of the model, while the latter permits it. Some models may have, for example, a foot on the bed not requiring support, but the arm directly above the foot does. "From the bed only" means the arm would droop/fail as no support would be generated above that portion of the model.

If you are using the ReplicatorG 0400 software, referenced from the manufacturer's web site, page 11 of the manual will have additional information regarding support within the program.