One of the knee jerk reactions I see a lot of DMs, GMs, Storytellers, etc suffer from is that when PCs are displaying unwanted behavior they want to fix it. To be clear, I'm not talking about cheating, harassing other players, or actions that harm the game when I say 'unwanted behavior' in this post. I mean things like murder hobo'ing, being overly aggressive with NPCs, not caring about social aspects of the game, etc.
You'll see posts in online communities about these situations like "my players just kill NPCs for their stuff regardless of whose side they're on" or "how do I stop my players from just being murder hobos."
I'm not saying this behavior isn't problematic for your game. However, what's going on here isn't "my players/player characters are broken." It's something else, and you need to find what that is. Some big things to check? Yeah, I got that.
Clash of Expectations
the big one is the expectations of the game. If you were looking to run a fantasy epic, and your players were looking to play a 'slay and loot' dungeon crawl...well, yeah you're going to have problems. Your expectations for the game are different, and so obviously you're going to approach the game in different ways and that's going to cause a clash.
The solution to this is a conversation. Ideally you have this conversation before the game starts - setting expectations for the campaign is part of the GM's job during setup for the game. However, you can have the conversation at any time. It may be worth talking to your players and be like "I feel like you guys are just walking around murdering people, and I'm trying to have this whole world. What's going on?" and just start the conversation. Maybe you start over. Maybe you fix it going forward. Note though, you're fixing the game, not the players.
Lack of Agency/Consequences
I'm lumping these together because they are very similar. If your players have no impact on the game's story - aside from moving your plot along to the next story beat - or there are no consequences for your players' actions you can end up with problematic behavior. It's kind of like how 'bad' kids will act out for attention, and I've noticed the desire myself in some games. Go enough times with no consequences for my actions, and no real agency in anything going on, and I'm going to start looking for something to break just to see if it gums up all the works or you'll flat out say nothing I do matters.
How do you fix this? Well, again, a conversation to start is good. Then start giving agency to your players. Make their actions matter. Make their actions have consequences. Have the world react to them, and have the world move on when they do nothing.
The Player Feels Ignored
This is similar to a lack of agency/consequences. Often when "I need to fix this PC" threads come up it's not the whole group - like what happens with the above 2- but just one player. And sometimes that player is acting out because they feel ignored. They have no story beats (that they're aware of) and have no role in the game aside from being another set of numbers in combat to kill monsters. Since combat is the only place they get to act, they make more things go to combat.
If you talk to a player, and they say they feel ignored do not try to point out all the things you've done to try and hook them. Those things failed, and while that is partly on the player it's not going to help. It can be hard enough for a player to say they feel you're ignoring them, if you respond with "well actually, I'm doing all this stuff but you never jump on it" they're just going to shut up and continue to feel ignored. Instead, take the proactive approach for the future. Don't worry about the past, just go forward "so how do we fix that?" "What would you like to happen?" "What kind of stories are you looking for?"
The initial responses are probably going to be "I don't know." Don't accept those. Approach the problem from the outside. Make some suggestions. Are they a fighter? Maybe a rival? Maybe a love interest? A personal quest? Work with them. Then point out where, when, and with what you're going to try to engage them next game. Tell them to look for it. And deliver.
To wrap this up, don't try to fix your players. Odds are it's something else in the game that is broken - not the player. Try to find the problem. Discuss it with the effected players. Work together to find a solution and go from there.
Don't just blindside a player with suddenly harsher consequences than the game normally has. Warn them first. Find out why if you can. Go from there.
Games are supposed to have fun. So find out how to get the game back to where it's fun for everyone.