It Follows- Ambiguous Ending

From https://www.vulture.com/2015/03/it-follows-director-the-spoiler-interview.html

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 visual.

MITCHELL: 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.