Like you, I am a writer who loves character conflict. There's nothing I love more than creating two characters who each have virtues and flaws, and put them at odds with each other. I want my readers to care about them both (I hate Bad Guy type villains, personally) and when I write I tend to put my story in an impossible situation. You want them both to win, but they cannot because their goals are diametrically opposed.
I'm also a writer who doesn't tend to do a lot of planning beforehand. When I'm really in the zone, it seems to me like I have absolutely no idea what my characters are going to do until they do it. I watch the story being written as the words flow from brain to pen to paper. I find myself occasionally appalled by what my characters are capable of doing, and I wonder "how are you ever going to pull a happy-ever-after out of THAT?"
As you may imagine, plot is my weakness as a writer. If I don't have a strong sense of destination, my characters will wander off into places which are interesting, but do not contribute to plot movement.
And I frequently find myself written into a corner; how can my heroine resolve this impossible situation? What can she possibly do to "fix" things? As I go back and analyze my own patterns, I can see that I have a particular theme that my characters use to solve the problem, and for me that is the concept of "self sacrifice." Once I had figured that out, it became a lot easier for me to figure out how to resolve those impossible conflicts.
I ask myself; what is the problem here? What is the actual heart of the conflict? What are the two forces which are clashing? Then I ask; how can my heroine sacrifice herself to make everything come out all right. I'm not suggesting that this be your theme, because you are not me and what pushes my buttons may not push yours.
But instead of asking "who should win", I would suggest that you focus on how each character drives himself forward into the story. Find a way to make both men learn something, both become better than they were.
It also depends on the age group that you are writing for. Teenagers look for things to be more black and white. They want Good Guys and Bad Guys. They are just starting to explore interpersonal mapping, trying to develop ideas about what is right and what is wrong. Read any YA novel and you tend to have your villains always being villainous for little reason other than that they are basically Bad and your protags doing good things because they are the Good Guys and that's what Good Guys do.
But I think older readers want more ambiguity. There are those who do good things and those who do bad things, but the roles tend to swap more. What is important to me in a story is not who is good and who is bad, but "who do I like?" If you create a villain who your readers like, how do you resolve that in ways that satisfy? My belief (based admittedly on my own personal taste) is for the events of the story to lead all the characters into change, into becoming the kind of people for whom there can be a solution that doesn't destroy anyone.
Each conflict is different. Once you decide how your characters need to change in order to satisfyingly resolve the conflict, it becomes a lot easier to shape the events so that the characters are molded into better persons.
And if you decide you want a more traditional good-guy-wins ending, then decide how your villain needs to be shaped in order to make him a person who deserves to win, and how to shape your hero so that he doesn't.
One other suggestion that I would make. Instead of having the character swear a dramatic "I will never kill again" oath perhaps you could, instead, have that character be shaped into the sort of person who thinks it is wrong to kill. It's a lot more work but IMO a more satisfying paradigm. And you can gradually corrupt your good guy until you come to the confrontation, where your former bad guy wins because (from a meta perspective) because he is now someone who deserves to win. There's also no reason whatsoever to kill your former hero. There are so many ways he can lose. Have his former allies discover how dishonorable he had become and change their alliances. Have him collapse into defeat at the realization that his opponent is the better man. Have him killed by the slimeball types that he has been surrounding himself with. The possibilities are endless!