This happened during my first blind playthrough. Actually I just wanted to record the ghost leviathan spinning in circles, but when I got too close, it attacked me. I fought back and noticed that I made it bleed which made me wonder if you can kill it. Turns out you can. Too bad you don´t get any resources from killing leviathans, but not having to put up with them for the rest of the game after you kill them is a nice reward too. Plus it´s a great experience :D