They did not come back for vengeance... (spoilers)

.. they came back for Elizabeth.

We tend to view remakes with too much baggage of the earlier films. For this version, If you look at the motives of the ghosts as seeking Elizabeth, rather than vengeance on the town, it can explain why they quit without finishing off all of the descendants of the founders.

Somehow the "first Elizabeth" became separated from the others either physically, by surviving and having children on the island, or just spiritually being reincarnated in an islander.

There are a few key scenes that seem to show this.

The last bit of the final flashback shows Blake reaching out to her -- as they sink -- and screaming when he cannot reach her.

The key scene is when Machen (the beachcomber) meets Elizabeth on the first night.
1) He says "you know the difference between flotsam and jetsam… some people wash up on shore, some bob around at sea for ever." - Elisabeth was the one "washed up" and the rest were "bobbing around".
2) Machen then says "you used to be Elizabeth"
3) He then warns her that If she touches the watch "things will change" - We can see the watch is not moving.
4) He hands her the watch, and it starts ticking - This starts the events - not the release of the items in the fishing scene. The fog only appears after this point, on a night where the forecast was for clear.
5) She asks "where'd you get it?" he replies "from the sea. It came back." She says "Sooner or later everything does"
6) She then realizes she has seen the hallmark before.

As the ghosts come back they are quite mad and destroy the towns people but that is really not their motivation. (OK, maybe some of it.)

Later as everyone takes shelter in the town hall, Elizabeth stops on the stairs and looks back at the fog. It appears at that point she knows they have come for her.

When she is reunited with Blake, everyone is made whole again -- the leprosy scars are gone from the ghost's faces -- the terror ends, and the fog dissipates. It is confusing because the expected "vengeance" is not complete.

At the end, Stevie asks "did itcome back for vengeance, or justice, or maybe to claim something lost long, long ago?"

I believe it was the latter.