In emacs, i have a small text selected as shown in the picture.
When i do eval-buffer, i like the kill-region to remove only the highlighted text. But this code empties the buffer.
;;this is a simple comment.... (message "%s" (region-active-p)) (kill-region (region-beginning) (region-end)) Doesn't region-end retun the selection region's end?
As the comments suggest, eval-buffer manipulates the position of point, so that by the time you call kill-region, region-beginning and region-end are no longer what they were when you started. The position of point is 'preserved', as @ColinBell points out, but in this case that means that point is moved during eval-buffer, and then restored to it's original value when it's done. So from the user's perspective, it doesn't change.
eval-buffer really doesn't make sense for code that is intended to modify the buffer itself. There's almost certainly a more appropriate way to do what you want.
eval-bufferis probably either selecting the entire buffer, but too fast to see with the naked eye; or, perhaps the point (region-end) is pushed to the end of the buffer so that the region envelopes everything.