There's not a specific final explanation, it was intentionally set to make you wonder. Here is an interview where the director of the movie was asked about the ending:
Initially, they do fail to dispatch the monster, but
after several gunshots to the head, the pool fills with the monster's
spreading blood and they can escape. At first, it's a relief — it
seems like the monster has finally been slayed — but as the blood
cloud keeps expanding outward, it becomes a more and more ominous
All I can tell you is that I've talked to people who have
read that as a conclusion — they see that sequence and believe that
the monster has been destroyed — and then there are other people who
see it and feel that it's a sign of their inability to destroy it, or
for it to be destroyed, period! I imagine people can figure out how I
feel about that shot, but I won’t say specifically.
It is naive to think you could kill something that is already dead, but they are kids so I guess that’s why the guy (who was obsessed with the gal) made also the stupid choice of risking his life just to get laid, but you can call him romantic, too.