Hill, 36, conceded that she was armed with a gun, a Taser, a stun gun, a sword and other weapons when she went to Selma Hill's home in the 7700 block of Peppertree Road in Dublin on Jan. 7, 2009, but said she acted in self-defense because the elderly woman was wielding a knife.
Rosa Hill said she went to the residence because her then-husband, Eric Hill, lived there with their daughter and she believed that he had been molesting the girl, who was 2 years old at the time.
Eric Hill has denied that he molested his daughter.
Rosa and Eric Hill got married in 2005 and had their daughter in 2006 but separated in 2007 and ultimately were divorced.
There was a custody battle over their daughter and a family court gave Eric Hill sole legal custody and 85 percent physical custody over her.
In her third day on the witness stand in her trial on charges that she murdered Selma Hill and attempted to murder Eric Hill in an alleged plot to take her child back, Rosa Hill said she saw her husband and daughter naked together in a compromising position when she climbed a tree to spy on them on Jan. 7, 2009.
Rosa Hill said that after her husband drove off, apparently to go to work, she approached Selma Hill when the elderly woman was taking out the garbage and tried to talk to her about her concern that Eric Hill was molesting their daughter.
But Rosa Hill said Selma Hill told her that her grandson "could do whatever he wanted to do" because he had custody and "if I interfered I would never see my baby again."
She said she was "shocked" and "really upset and angry" about Selma Hill's comments and she hit Selma Hill after the 91-year-old picked up a knife and "it seemed like she was swinging the knife toward me."
When prosecutor Casey Bates asked Rosa Hill why she did not just take two steps back and leave, she said, "I wasn't thinking at the time. It never crossed the mind."
Sobbing on the witness stand at times, Hill said she was frustrated that the family court disregarded her allegations that her husband was molesting her child.
She said "I completely lost it" after Selma Hill's comments to her.
"I just started seeing red and I couldn't see anymore," Rosa Hill said.
She said, "At some point I think I Tased her and I think I shoved her back really hard with the broom and she started falling," Hill said.
Hill said Selma Hill started gagging and lost consciousness.
She said she did not know whether the elder woman was dead so she used a stun gun on the 91-year-old "to see if she moved."
She said she finally realized that Selma Hill was dead "after I used the stun gun on her a few times and she didn't move."
Hill said she did not call police because she did not know how to handle the situation and was concerned about what would happen with her daughter.
Hill said she stuffed Rosa Hill's body into a garbage can because she did not want her daughter to see what had happened.
Hill's mother, 57-year-old Mei Li of Antioch, is also charged with murder for Selma Hill's death and with attempted murder for an alleged attack on Eric Hill later that day after he returned to his grandmother's home.
Bates contends that Li was there for the attack on Selma Hill, but Li's attorney claims she only arrived at the home later, after her daughter called and asked for her help.
In his opening statement in April, Bates alleged that Rosa Hill and Li planned the attack at Selma Hill's home for months, buying two guns, Tasers, stun guns, a hammer, a sword, a crossbow, an axe, a hacksaw, handcuffs, leg irons, pepper spray and other weapons.
But Rosa Hill's lawyer, Bonnie Narby, said she does not think the alleged attack was a murder because Hill never intended to kill Eric Hill or his grandmother.
Instead, Narby said she thinks Rosa Hill acted in the heat of passion because she was overwhelmed by the custody battle over the couple's daughter and their divorce proceedings.
Rosa Hill insisted today that she acted alone when she killed Selma Hill and said her mother only came to the house later.
But in a hearing outside the presence of jurors, Bates said he believes Rosa Hill and her mother "acted together and were both active participants before, during and after the death of Selma Hill."
Bates said he also believes they acted together in a plan to kidnap Rosa Hill's daughter and drive off in a getaway car.
Bates will continue his cross-examination of Rosa Hill when the trial resumes Tuesday morning.
Rosa Hill faces a term of 44 years to life in state prison if she is convicted of all the charges against her, and Li faces 38 years to life.
Ping Li, 70, the husband of Mei Li and the father of Rosa Hill, is charged with being an accessory in the case but will be prosecuted separately at a later date.