I wish I could remember the name of a series I would recommend you look at it. It was several years ago. A group of the main characters were convinced that the leader of their family was planning actions that would destroy their way of life and for the sake of the family they felt he had to be killed. And they were determined that they should be the ones to do it because they loved and revered him the best. Which made for a lot of angst amongst the conspirators, especially those closest to him. From their viewpoint, of course, he was the betrayer by following a course which they were convinced threatened the welfare of their large extended family, but they also felt very keenly the personal betrayal that they were planning. So I remember feeling very sympathetic to their intended victim (because he was an idealist and what he planned didn't sound one bit wrong to me), but also to those who plotted against him, there was so much anguish about what they felt they had to do. The tension had me pretty much tied up in knots. After the first book, when the plot went awry, and some of the conspirators were on the run, it was still pretty angsty, but not to the same extent
Dang! I wish I remembered the title. There was a river and a lot of chapters devoted to travelling on boats.
Anyway, the point is, that it is possible to have "betrayal" going both ways when people have different ideas about what is the right thing to do.
But if you want a more traditional bad guy type traitor, historically the motive usually is money, power, or revenge. Money and power/position are the most convincing motivations, since revenge, though certainly no less realistic, can veer, if a writer isn't careful, too far toward melodrama. If the traitor is being offered a chance at all three, of course, he will be very strongly motivated.