Bandits fighting to the death always strikes me as profoundly stupid.
The moment they realize they're outmatched, they'll flee. If they're more evenly matched, they're still going to flee when it doesn't go their way.
Bandits steal to survive, kill when resisted when they can get away with it, and most importantly regular bandits will not attack a clearly fully geared, armed, and armored party of 6
If I'm the DM if regular unaffiliated bandits are attacking, there's a "reason" behind it that makes sense other than "oh no random highwaymen"
It's a minor pet peeve but I'm glad I'm not only person who thinks about realistic encounters.
Except the anti-metagaming tarrasque. That's an extra dimensional being who doesn't follow logical rules. Like meta gamers.
There's a lot of narrative potential there. Why would a group of rag-tag bandits attack a squad of seasoned fighters? Perhaps they were misled by their cruel leader who tortures those who fail to "motivate" the remaining bandits.
The leader could be secretly backed by a powerful noble who is funding the bandits as a way to remove competition. The leader doesn't need the bandits, so their lives are expendable, but the bandits are the outcast, down trodden, and desperate who have no alternatives.
What seems to be a simple case of greedy bandits suddenly becomes a power struggle between noble houses with the backing of the crown. Killing the leader may solve the issue in the short term, but the nobles will just install a new chump to continue the plan. Killing the nobles will have greater consequences across the kingdom. Disposing the crown will throw the kingdom into chaos.
And it all starts with one, desperate highway man begging for his life in the mud while the party thinks "it's just a bandit, they're evil, so they deserve to die."
One of the most amusing and at times frustrating games I ever played was a campaign inspired by the Napoleonic Wars. The French were Gnomes who were extremely bad French stereotypes armed with muskets and liable to flee if their unit got attacked by three volleys OR their officer and/or standard bearer got sniped. Several parlays included awful french accents on the part of the DM and more than one encounter outright avoided by the PCs hiring a bunch of whores to distract or appease the frognomes, complete their objective and move on.
In a high level campaign I ran, I took the design philosophy that the villains were supernatural (e.g, dragon or lich), the average npc was weak (level 3 or less), and the characters were once-in-a-1000-years heros (level 10-20).
Every now and then they would have an obstacle involving regular humanoids or the local government and they had the option of just steamrolling everything (even whole platoons). It provided a great contrast to the magic-boss death matches and let the characters really feel special.
It also drove home that they were the only ones who could save the day.