How to handle overly aggressive Characters ?

I’m fairly new to D&D, and I also DM a campaign with a bunch of my close friends and even siblings. I completely understand why you would want to be delicate about this- I do, too. Maybe there is a way to be kind about it, but still firm. 

“Hey, guys. Recently, I’ve been struggling with DMing a lot because I feel like the plot is stunted when you guys kill my NPCs. Would it be okay if you could tone it down, so we could have a lighthearted game?”

They may not listen, so that’s when you may want to consider in-game consequences. Maybe have a bounty put on their head. A bounty hunter could hear about them, for example, and chase after them, sort of like the Mandolorian. This could put pressure on the character to change their alignment, and I would suggest talking to them one on one about the possibility of that.

If they still don’t listen, start small by removing them from one session. Be kind about it, of course, but real friends should be a little firm with each other, sometimes. If they have an issue, talk it out with them, but don’t take away the consequence. If the behavior continues, tell them you are considering removing them from the campaign. If they still don’t stop, then do so.

I know it’s hard, but a fun game between you and you’re other friends is absolutely worth it. Good luck!