bad blood (n.)

• acrimony
• animosity
• animus
• antagonism
• antipathy
• bitterness
• collision
• competition
• conflict
• contrariety
• contrariness
• dissension
• enmity
• feud
• fractiousness
• friction
• hostility
• malevolence
• negativeness
• obstinacy
• perverseness
• rancor
• recalcitrance
• refractoriness
• repugnance
• rivalry
• soreness
• sourness
• vendetta
• venom
• virulence
• vitriol
• whammy