To avoid this...
The slower you level, the more scavenging/looting/building time you have relative to your gamestage*. Gratuitously going after zombies for the xp (rather than just killing them in self-defence) - "power leveling", as Hrod Land calls it - inflates your gamestage more quickly than is necessary and you're just making the game harder for yourself.
Refraining from killing every zombie in sight doesn't have to be a role-playing thing. It can also be a strategic thing.
*Actually, leveling too slowly can also make the game harder, because the gamestage will tick up over time anyway but you won't yet have access to the higher level perks.
Basically, level too fast and you'll be underprepared. Level too slowly and you'll be underpowered. There's a happy medium somewhere where you have enough preparation time and you've also got enough perks, but the exact balance of where that will be is going to depend on play style and skill.
The gamestage calculations have a built-in cap to prevent you leveling so slowly that things get out-of-hand and your game becomes no longer playable, but there's no cap the other way. Providing you can still keep killing zombies and leveling, you can keep making it harder for yourself.