Judge exposure by the histogram, not the rear LCD preview. The camera screen can easily mislead you if its brightness is set too high or if ambient light changes, so a JPEG preview that looks bright doesn’t necessarily mean the RAW is overexposed.

RAW files also often look flatter and darker than the camera’s processed JPEG preview, because the JPEG has in-camera processing applied while the RAW does not. So some adjustment in RawTherapee is normal.

That said, if the histogram shows the data bunched toward the left, your RAW file is indeed on the dark side. Since you mentioned using negative exposure compensation because the preview looked too bright, your LCD brightness may be set too high. Check the camera’s LCD brightness setting and avoid judging exposure by the screen alone.

In short: some brightening in post is normal with RAW, but use the histogram as your main guide to decide whether the image is actually underexposed.