The ex-wife of Staten Island decapitation victim Anthony Casalaspro says the doomed city sanitation mechanic had “big plans” to spend his retirement in Florida — but his girlfriend’s son, who cops say brutally ended his life, didn’t want the troubled family to leave New York.

Casalaspro’s girlfriend’s son, Damien Hurstel, 19, is accused of stabbing him to death in their Cary Ave. home near North Burgher Ave. in West Brighton Monday afternoon, chopping off his head and leaving the knife sticking out of his neck in the blood-soaked bathtub.

“This is horrendous,” the victim’s ex-wife Valerie Casalaspro told the Daily News in an exclusive interview. “I’m shattered.”

Anthony Casalaspro, 45, tried to do right by his accused killer, she said.

“He was literally just at the kid’s graduation f

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