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The main character Steve decided to be a police to stop the corruptions he saw and change things around him. Do you think he did what he wanted to do? How?

Steve was unpopular with his fellow officers and his friends also distanced themselves from him. Though he had a hard time of it, he remained loyal and honorable to his cause and did not fall prey to the corruption around him. He was successful in his endeavors as toward the end of the novel he received a package from Roxy with headlines about the crackdown on corruption, arrests of police officers made and the suicide of the police chief, McGuire.

At first, he thought and believed that he failed in his mission.
But as the book comes to an end, it's unarguably true to say that the change he wanted did occur because all the corrupt guys were dealt with.

Steve achieved what he planned to in the police force. Despite the hardships he faced during the short period of time he was there, he was able to differentiate himself as a good cop and the ground work he did before he exited the force brought about the results we saw at the end even though he had left the force.

Well in a way, he did stop the corruptions. But, we don't know if it all truly stopped or was just replaced. Steve had to watch passively as a lot of wrong things were going on, and that must have been very frustrating as he wasn't in a position to stop it all from happening.
Sure, Steve brought down the major players in the corrupt force, but we don't know for sure that the ones left behind did things differently or were better. So, I honestly don't know if he succeeded. But Steve probably felt he succeeded in making a change for the better.

I think he tried his best, though towards the end of the book he felt like he killed someone and was very disappointed with himself but to some extent he was honest and he avoided killing as much as possible

Can't pinpoint how but I know he did some of the things he had in mind. It's not so easy to change a corrupt system, especially in these kind of situations.

Apparently, he did, as we saw from the journal clippings that Roxy sent to him. I don't know if it was what he truly wanted to do, but he succeeded.

Steve was who he wanted to be, but didn't achieve what he wanted at last.
And if we are going to generally define a "good police officer", Steve wasn't one. Steve was someone with quite a good heart, but he didn't apply law strictly; he casually had drugs at home when that was illegal, and what scared him is just getting caught. However, he did his job better than any other officer who appeared within the story.
And about achieving his grand goal, he obviously didn't, and the end they all got was mostly fate. With all due honesty, changing a whole system all alone when you're just an officer wouldn't be realistic. So the end was the best result possible.

I don't think Steve was successful as a police officer. Although he was able to help with investigations to bring down corrupt officials, I think he would have been more successful as a lawyer.

I don't think he fully achieved everything he wanted to do. I think he did make some differences happen but not to the extent he might have wished for.

He managed to do his thing despite all the danger and the corrupted system. So he was successful.

He accomplished his mission. The job was just the best for him.

I actually think Steve was very successful in his police career. In fact, that was just the best job for him

Yes. The resolution of the story shows that the top guys involved in corruption are arrested. That's a big win for Steve as a cop trying to change the world.
What he wanted was to change the narrative of corruption in his city and I think he achieved that, though not absolutely. I don't suppose corruption will vanish with the arrest of the top cops. The fraternity will likely replace them with another set of people. That's how corruption rolls. It's not something that can be dealt with once and for all. It's a process. But I think people like Steve will rise in the department with the appointment of a new head.