The 23-year-old ex-lover of a former New York City schoolteacher killed her and their 4-year-old son because he was angry in part that she was dressing the boy in girl's clothing, police said Wednesday. 

Isaac Duran Infante, who was arrested Tuesday in the deaths of 36-year-old Felicia Barahona and their 4-year-old son Miguel, allegedly confessed to police that he first planned to kill Barahona but then ended up killing the child as well. 

NYPD Chief of Detectives Bob Boyce said at a news conference Wednesday that the mother "did dress the child in girl's clothing, and he felt that antagonized him."

"But there was other issues as well, there was monetary issues," said Boyce. "He did have a new girlfriend in Bethlehem, Pennsylvania, where he lived." 

Police said surveillance video captured Infante outside Barahona's apartment door in Hamilton Heights last Thursday, then leaving that night, after allegedly killing the woman and their son. 

"We have him on video in the hallways pacing back and forth in front of the apartment," said Boyce.

A neighbor of Barahona told NBC 4 New York he noticed a foul smell coming from upstairs last Friday. The building superintendent finally went into the apartment early Monday morning with police, and that's when the bodies were found -- Barahona in the living room with an electrical cord around her neck, Miguel in the bathtub. Infante had strangled both of them by squeezing a cord around their necks, according to a criminal complaint.

Infante was arraigned in Manhattan Criminal Court Wednesday on two counts of second-degree murder and one count of first-degree murder. His attorney, Allen Farbman of Legal Aid, said his client was "very upset and crying" before the arraignment. 

"It's a very upsetting case, obviously," said Farbman. And despite the prosecutors' assertion that Infante had confessed to killing Barahona and their son, Farbman said "I haven't seen any confession, I don't know anything about it. It's very difficult for me to comment on a confession that the D.A. said was made." 

Barahona was a former science teacher of Infante at the DeWitt Clinton High School in the Bronx, and the two met and started their affair while he was still a student. Barahona lost her job in 2013, shortly after she became pregnant and school administrators learned of her illicit relationship with a student. 

His anguished grandmother, who lives in the Bronx, told NBC 4 New York in Spanish she wanted justice and that she knows her grandson is innocent of the charges.  

Another relative of Infante, Janiel Santana, said, "It's hard to believe they're dead. When we heard the news, we thought this has got to be a mistake." 

The couple shared custody of the child, relatives said. Infante's family was expecting to see the boy for New Year's Eve to give him his Christmas presents. 

Infante has been remanded back to jail without bail and is next scheduled to appear in court Friday.