One who instills hostility

• Causer of estrangement
• One who turns away a relation in trouble?
• Turnoff
• An unpleasant person who causes friendly people to become indifferent or unfriendly or hostile
• Unpleasant sort
• Opposite of 20 Across

Wiktionary Word definitions in Wiktionary
n. A person who alienates

WordNet Word definitions in WordNet
n. an unpleasant person who causes friendly people to become indifferent or unfriendly or hostile

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Alienator \Al"ien*a"tor\, n. One who alienates.