Ending of It Follows 2014

EVC's answer covers most of it, but there are a few other in-universe points that I think are worth adding, and I kept going over the comment character limit. Spoilers follow.

While we know the ending is intentionally vague, we have a few other things to consider:

1. The Follower is not completely stupid. It will pick up rocks to smash windows and create entrances, and throw things rather than walk into an obvious trap.
2. The Follower can be hurt. We don't know to what extent it can be harmed, but it opted to throw appliances at Jay rather than just hop in the pool to get her. It also hits Paul in retaliation for getting smacked with a chair, even though he wasn't its target, and it later holds its hand up in the way of the gun when he's trying to shoot it at the pool. On top of all that, we see that it gets knocked down by glancing shots to the head and needs a moment to heal/recover.
3. The Follower can create "props". At one point, it manifests as a urinating woman, and leaves urine on Jay's kitchen floor. If we combine this information with points 1 and 2, then the blood at the end might be something it's intentionally creating as camouflage, like a squid squirting ink. It may need to "play possum" until it can recover.
4. Paul isn't just taking the curse at the end. In the final shot, we see Jay and Paul holding hands as they walk down the street, implying that they're in an actual relationship. Jay didn't sleep with him to pass on the curse, she slept with him because she thought the curse was finally broken.
5. It might still follow. In the final shot, we see someone walking towards Jay and Paul in the background. The person is in ordinary street clothes, and there have been enough "false alarms" throughout the film to give the viewer a sense of uncertainty about this final shot.
6. The Follower isn't a ghost. I only mention this because the original question says "It is naive to think you could kill something that is already dead". We don't know what the Follower is, but we have no reason to believe that it's a ghost or any other undead entity, based on anything in the film. It could just as easily be a demon or a curse.

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 visual.

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.