Curbing Murderhobos
If your players have some murderous tendencies, the first step is always to talk with them about what kind of game you're running vs what kind they want to play, and make sure expectations are aligned.
If the party all want to play a rambling gang of thugs, and you're down to run such a game, then murderhoboing can be fun! But if you prepped a game for heroes who want to do good, or the murderhobo is ruining fun for other players, it's time for a talk.
That said, here are some houserules I use to clearly signal that my game is not set up to accommodate murder hobo-ing.
Innocents Yield Negative XP
If you're using the XP system, this one stops the temptation to murder realquick. My loose definition of innocent is "someone who did not pose a threat."
Some players don't mind if their character is in trouble or disliked in the world. Negative XP gives a meta-level consequence that they might be more deterred by.
In-World Consequences
Speaking of being in trouble, your world has law enforcement, does it not? Characters who murder, rob, intimidate, harass, or otherwise engage in criminal behaviour should have wanted posters out. Wanted posters mean bounty hunters.
Being wanted also means that there's suddenly no vacancies at any Inn you visit, and the places that will serve you charge double, because they know no one else will.
Beefier Guards
It's easy to laugh off the law enforcement if you know that a guard is only CR 1/4. So homebrew yourself up some guards. Give them better armour, more hit points, spells, and extra abilities. Runesmith has a great video for this.
At the very least, guards should be able to inflict the restrained condition (nets and grappling abilities are easy ways to do this) and call for reinforcements (give them a reaction that does this). Give them spells, too. Hold Person is such a classic guard spell. Same with Calm Emotions.
Most Things Run Away
Few creatures are actually motivated to fight to the death. The exceptions are mindless things like oozes, cultists and the like who think they'll be killed anyway if they flee, and... I can't think of a lot else, tbh. Most stuff wants to live to fight another day.
The Monsters Know What They're Doing includes an estimation of the point at which a monster tries to flee in every entry. Remember it when you're running combat. It's a lot harder to insist you're playing a hero if you keep killing people who were surrendering or running away.