Greatwyrm
Been here a while...
To start, if you haven't read Play Dirty by John Wick, I certainly recommend it. A revised version, as well as a second volume, should be out soon-ish.
Second, you need to remember that it's your job. You have to be the bad guys, because it's the DM's job. Your players, I assume, want to be heroic and need bad guys to squash. You are being good to your players when you give them someone worth hating to struggle against. You can mow down random kobolds all day long and no one will talk about it a week later. That kobold shaman that wore a necklace of stolen wedding rings and poisoned that tiny village's fields just out of spite? That's a guy they want to rough up.
Finally, avoid outright killing them when you can make them suffer. Practically everybody has their next character concept in progress before the ink is dry on the one they just rolled up. Death isn't scary at all. (I'd also avoid killing PC family members or other loved ones.) Burn the village and make them homeless. Foul the crops. Slaughter all the cattle in the night and leave the bodies to draw in wolves. Spread vicious rumors in the city. Frame them for crimes. Make them live under a legally-appointed tyrant for a mayor or sheriff. Unfortunately, you can pick up any newspaper or history book an find terrible people doing terrible things to other people. Your bad guys need to do that stuff (within the limits of what your group accepts). Your players will moan and wail over all the terrible things you're doing to their characters, but I bet they keep showing up for the chance to squash that kobold and be the heroes that give all the rings back.
Second, you need to remember that it's your job. You have to be the bad guys, because it's the DM's job. Your players, I assume, want to be heroic and need bad guys to squash. You are being good to your players when you give them someone worth hating to struggle against. You can mow down random kobolds all day long and no one will talk about it a week later. That kobold shaman that wore a necklace of stolen wedding rings and poisoned that tiny village's fields just out of spite? That's a guy they want to rough up.
Finally, avoid outright killing them when you can make them suffer. Practically everybody has their next character concept in progress before the ink is dry on the one they just rolled up. Death isn't scary at all. (I'd also avoid killing PC family members or other loved ones.) Burn the village and make them homeless. Foul the crops. Slaughter all the cattle in the night and leave the bodies to draw in wolves. Spread vicious rumors in the city. Frame them for crimes. Make them live under a legally-appointed tyrant for a mayor or sheriff. Unfortunately, you can pick up any newspaper or history book an find terrible people doing terrible things to other people. Your bad guys need to do that stuff (within the limits of what your group accepts). Your players will moan and wail over all the terrible things you're doing to their characters, but I bet they keep showing up for the chance to squash that kobold and be the heroes that give all the rings back.