He was fed up with killing before it started. He came to america to get away from it. He had a secondary goal of performing one final kill to finally put the past behind him, but that's about it. But then things got bad and he had to go back to his previous way of solving problems for lack of alternatives.

I think he definitely changed his opinion on seeking revenge, but he always seemed to give an air of not wanting to commit the crimes he did. He really just lacked a choice. I don't see how the ending gave him a way out any more than the beginning did. Sooner or later, I see things going poorly for him again.
He gets married, settles down, and has a family. He's quite happy and no longer has to rely on Niko for everything. Niko/Roman have a friendship, but it's also a serious dependency. Niko feels sorry for Roman due to a couple of things (the situation involving his aunt comes up at one point), and Niko generally a way for Roman to solve his problems (which appeared to be at least part of Roman's invitation). Niko depends on Roman to relieve his guilt and as a sort of lifeline to his lost humanity (Niko is far more "normal" when around Roman, and sometimes seems happy. Once Roman is out of the picture he's generally seen brooding about something or committing a crime (often for Roman's sake)). Packie seemed to be about the only character Niko spent considerable time with without much of an motive or reason. He just opts to spend time with him (perhaps in part due to them being somewhat similar. They both had fucked up childhoods, they both have to resort to violence to get by, and they both have a family member they go out of their way to protect). Helping him didn't seem to have quite so much of a purpose as the other missions generally did. No blackmail, no huge cash revenue (at first), no saving roman.
Ironically, Kate dying would probably have the same effect on Packie that Roman's death would have on Niko. You're just choosing which character to ruin.
