>>34662624Nothing stopping them but your imagination. If you characterize your villains a particular way they may have reasons to hold back or not fight tooth and nail. Then it's your responsibility to develop a rationale for their behavior. Plenty of tropes to choose from: past trauma, sympathy for the hero, faulty convictions, or, as you said, a sliver of honor or simply underestimating their opponent.
As for who breaks the rules, think your classic bond villain. Anyone who stands to lose everything if they don't get their way. It's not an exhaustive list however. You can create motives where you need them.
On a more meta level, consider how your readers will react to a final showdown where conventional rules are followed. YMMV depending on what it is you're writing. To make for a more interesting conclusion, it might be of some benefit to your work by having an intense final stand which departs from conventional rules. Trust your literary instincts.
