General:
* Animosity
* Antipathy
* Aversion
* Bad blood
* Bitterness
* Discord
* Enmity
* Hostility
* Resentment
* Spite
* Tension
* Unfriendliness
More specific:
* Grudge: A persistent feeling of ill will.
* Rancor: Bitter resentment.
* Vengeance: A desire for revenge.
Informal:
* Bad vibes
* Beef
* Friction
* Bad rap
The best synonym to use will depend on the specific context and the level of formality you need.