The Sheriff's office said Bruce snr had left his wife and moved with Bruce jnr to Hobbs from Oklahoma after allegations she had been abusing him.
They said he planned to divorce her to protect his son but had allowed her to stay one night after she begged to see the boy.
A man who lived near the Johnsons in Oklahoma said Mary had been exhibiting strange behaviour.
Erick Wyatt told local news outlet KXII that his daughter went to school with Bruce jnr and the whole community had noticed Mary's odd behaviour.
"Some of her comments talking about she's Mary Jesus and just different things, talking about hurting other people," Wyatt said.
"It kept on escalating more and more."
Her Facebook is filled with bizarre rants, including references to "stabbing a partner's heart … then yourself" and messages saying she "doesn't want kids".
Mary Johnson remains in hospital and police have issued an arrest warrant for first degree murder.
SUICIDE AND DEPRESSION Where to get help: • Lifeline: 0800 543 354 (available 24/7) • Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7) • Youthline: 0800 376 633 or text 234 (available 24/7) • Kidsline: 0800 543 754 (available 24/7) • Whatsup: 0800 942 8787 (12pm to 11pm) • Depression helpline: 0800 111 757 or text 4202 (available 24/7) • Anxiety helpline: 0800 269 4389 (0800 ANXIETY) (available 24/7) • Rainbow Youth: (09) 376 4155 If it is an emergency and you feel like you or someone else is at risk, call 111.