There is no good or bad Sawyer, both Sawyers have a good and a bad side (just as every character does).
Island-Sawyer never returned from the island, he's been there ever since the plane crashed. Cop-Sawyer on the other hand never crashed on the island, because in his timeline there is no island (and thus no Swan hatch to cause the plane to crash).
The reason alternate Sawyer is a cop is because the island was blown up in 1977 (causing the split in timelines), and therefore the characters' alternate lives weren't influenced by Jacob. Since Jacob wasn't there in the alternate timeline to give young Sawyer the pen to write his revenge letter, he eventually became a cop rather than a criminal (although this doesn't mean he's now good instead of bad, as evidenced by the fact that he still wants revenge).