Betty Broderick's son is trying to process his grief now that she's gone ... because, while she obviously killed his father, he also thinks of the good times.

Daniel Broderick -- the son of Betty and Daniel -- tells TMZ ... obviously his relationship with his mother became complicated after she killed her ex-husband and his new wife in 1989.

That said, Daniel also tries to remember the good times ... saying she was an amazing mom back in the day -- engaged, intelligent, fun and funny.

Despite his mom's actions, Daniel says he and his three siblings -- who were all at Betty's bedside when she passed -- loved both of their parents ... and, they've tried to find a way to move past the tragedy for their own sake.

What Betty did is unforgivable, Daniel says ... but she's also their parent. The siblings aren't absolving her of responsibility, but they need to move forward for themselves.

We broke the story ... Betty passed away at a California medical facility on Friday where she was transported last month. She was in the middle of serving a 32-year to life sentence. Near the end of her life, Daniel tells us Betty suffered from a number of septic infections ... and broke several ribs during a fall in prison.

Betty killed Dan Broderick and and his 21-year-old assistant, Linda -- with whom he had an affair and then married -- in 1989 after stealing a key to their house from her daughter. She was sentenced to 2 consecutive, 15-year-to-life terms.