He was just 19-years-old at the time of the shooting back in February 14, 2018, when he murdered 17 people and injured 17 others.
The gunman was caught on CCTV having a McDonald's meal with a student's sister just moments after his deadly massacre.
The deeply troubled man, now 23, had a history of behavioural and mental health problems and a deep love of guns, according to people close to Cruz.
According to authorities there were several attempts missed to try and help Cruz including when his biological mum left him after he was born.
The drug-taking mum was not a part of his life as he was adopted by his parents.
Cruz had shown aggressive behaviour in school and was diagnosed with ADHD, autism and obsessive compulsive disorder.
His life started to spiral when in 2004 Cruz found his dad dead at home after dying of a heart attack.
While at school in 2016 he revealed to a student he had a gun at home and was exploring the possibility of using it.
After learning of this information, two counsellors and the sheriff's deputy said he should be taken for psychiatric evaluation - but it never happened.
He found out he was adopted when he was 15 and his mum died shortly after of pneumonia in 2017.
And despite the warnings of wanting to kill others and making threats to students, Cruz's behaviour was never picked up on.
Below are 7 key revelations from the chilling sentencing trial.
The killer's mum 'poisoned' him in womb
Cruz's defence was to try and provide mitigating factors which would lessen his sentence - such as not getting the death penalty.
They began with calling witnesses which testified his biological mum Brenda Woodard drank alcohol and took drugs while pregnant with Cruz.
His team argued he suffered foetal alcohol spectrum disorders (FASD) as a result of her actions.
The killer's biological half-sister Danielle Woodard gave evidence to court which stated Cruz was affected due to coming from their mother's "polluted womb".
She told jurors she saw their mum drink, smoke crack and smoke cigarettes while pregnant with him.
Although the pair share the same biological mum they had separate childhoods as Cruz was adopted by Lynda and Roger Cruz.
Danielle lived sporadically with their mum over the course of her childhood.
Melisa McNeill, Cruz's lawyer, told jurors his brain is “irretrievably broken” as a result of his mum's booze and drug abuse.
She said: “His brain is broken. He is a damaged human being.”
The alcoholic mum was a drug addict, homeless and working as a sex worker when she became pregnant with Cruz.
Ms McNeil told the court: “His prenatal vitamins consisted of... Bum wine, crack cocaine and cigarettes.”
Cruz's dad died in front of him
Adoptive dad Roger Cruz was at home with his family in August 2004 when Nikolas, who was 6 at the time, ran to his mum Lynda crying.
She asked him if he was upset because his dad at shouted at him when he responded: "No, Daddy is dead.”
Lynda rushed into the living room and found her husband dead on the sofa after suffering a heart attack. The mum was left to bring Cruz up alone.
Cruz was set to attend a grief support group aimed at helping him come to terms with the traumatic experience.
However, it was revealed in court Cruz did not attend the support group until four years after his dad's death.
Warning signs issued by teachers ignored
Cruz's behaviour at school was alarming many including staff at the different schools he attended.
At the age of three he was diagnosed with developmental, mental health and behavioural issues and changed schools frequently in order to try and help provide better support.
During his time at West Glades Middle School, Cruz's eighth grade teacher showed the courtroom notes where she said he posed a risk to those around him.
She wrote on a behavioural assessment form at the time: “I strongly feel that Nikolas is a danger to the students and faculty at this school.
“I do not feel that he understands the difference between his violent video games and reality.”
It was revealed in court during an essay assignment Cruz had gone into great detail on his plan to create a "sex boat" which featured lurid drawings.
During another incident, a teacher once congratulated Cruz on his work when he responded: "I’m a bad kid. I want to kill.”
Ms Yon told the juror she once wrote that he needed to be in “a special school with smaller classrooms and locked doors”.
Cruz once asked teachers in a lesson if people "ate" the bodies of dead people during the Civil War.
It was during the same lesson on 11 September 2013 he "became fixated on death and assassination of Abraham Lincoln”.
The teacher told jurors: “Some questions he asked were ‘What did it sound like when Lincoln was shot?
"Did it go pop, pop, pop really fast? Was there blood everywhere? After the war, what did they do with all of the bodies? Did people eat them?’”
Letters written by Cruz said he "dream of killing"
A letter from a school therapist, Dr Brett Negin on June 5, 2014, was shown to jurors which detailed a session claiming he had “dreams of killing others and is covered in blood”.
During this period he was at the Cross Creek School in Broward County, Florida - a school for kids with behavioural challenges.
The therapist claimed he was “very irritable and reactive” at school and at home and had “a preoccupation with guns and the military and perseverates on this topic inappropriately”.
He would also “punch holes in the walls” if he ever got upset.
School mental health experts suggested Cruz's medication needed to be "reassess" and wrote a letter to his doctor. However, Dr Negin said he never received the letter.
He was under the care of Dr Negin from January 2012 to August 2017 and was diagnosed with Disruptive Behaviour Disorder.
Cruz went on toad "killing spree"
According to one former neighbour, Cruz once went on a "killing spree" of all the toads in his road as revenge for the family's dog dying after eating a toad.
Paul Gold, who lived next door to the family in Parkland from 2008-2011 and remained in contact with them, told jurors: "He tried to kill every toad in the neighbourhood.”
He would lose his temper and break items during his "episodes" as Cruz seemed like “almost like it was another person who had done it”.
Cruz's mum told Paul she was "scared" of him and would be afraid of his ability to turn angry quickly.
Mr Gold said: “She told me she was scared of him. She told me not to believe the nice appearance he had and angelic ways and that he would turn and do bad things. And she was a little afraid of him at some times.”
He told jurors when Lynda died in November 2017 Cruz was "very upset" at the small turn out for her funeral.
Cruz moved into Mr Gold's ex-partner Rocxanne Deschamps home just a month after but was forced to leave after falling out with her.
Expert 'never seen' so much prenatal alcohol abuse
A leading expert in foetal alcohol spectrum disorders (FASD) testified he had "never seen" so much alcohol abuse by an expectant mother.
Dr Kenneth Jones told the court: “I don’t think I have ever seen — I know I have never seen — so much alcohol consumed by a pregnant woman.”
The Dr said he worked with over a 1,000 people who were exposed to booze in the womb during a 50-year career and hadn't seen as much as hers.
It led to him stating Cruz "without any question" was someone with FASD after an assessment showed he had alcohol-related neurodevelopment disorder (ARND).
Dr Jones told a stunned court that FASD can “in and of itself cause people to commit murder”.
Another FASD expert was called to give evidence and Dr Paul Connor carried out neuropsychological testing on the killer and found he had nine out of eleven deficits in the assessment.
He added any person with three or more deficits is consistent with having an FASD.
Cruz looked for child abuse imagery and Nazi items
The internet search history of Cruz was investigated prior to the 2018 shooting showing he searched for Nazi paraphernalia and Hitler’s birthday.
He also searched for child abuse imagery and spoke of his desire to kill black people and used racist language online.
He had a fascination with Nazis as jurors were shown the military-style boots he wore during his attack which had a swastika on them.
During Cruz's time spent in his cell ahead of the trial he wrote notes saying he enjoyed causing “pain and death” to the victims he killed.
He also said to police how he liked killing animals when he was young including skinning and burning lizards alive when he was four-years-old.
Forensic Psychiatrist Dr Charles L Scott diagnosed Cruz with antisocial personality disorder - sociopathy.