Taylor Parker sobbed and insisted she's "a good person" ... as she's being interrogated for murdering Reagan Simmons-Hancock and stealing her unborn daughter, Braxlynn, from her uterus.

In interrogation video obtained by TMZ -- from just days after Raegan's murder -- Taylor cried to cops, saying she will "never understand it."

Investigators got Taylor to walk them through the night before the murder ... she says Raegan knew wasn't actually pregnant and was helping her figure out how to prevent her boyfriend, Wade Griffin, from catching her in the lie.

You can see in the clip ... Parker says she was at Raegan's house while they were brainstorming, and Raegen even offered to fake a call to tell Taylor she'd had a miscarriage.

But sadly, things went a very different way.

As you know, prosecutors accused Taylor of murdering Raegan -- who was over 8 months pregnant -- and cutting her unborn baby out of her in a makeshift C-section.

Prosecutors say Taylor was planning to pass Raegan's baby off as her own after faking her pregnancy for months. The stolen baby was pronounced dead shortly after Taylor was discovered by cops.

Taylor was convicted of capital murder and sentenced to death. She's still awaiting her execution date and is the youngest woman on death row in Texas.