Lindsey Glass, 27, is charged with allegedly violating a Texas alcoholic beverage code titled "Sale to Certain Persons" following Spencer Hight's murder rampage in September 2017.
Hight, 32, finished drinking at the Texas bar Ms Glass worked at before driving to a nearby home and killing the eight people at a football-watching party inside, including his ex-wife Meredith Hight.
Under Texas law, a person is guilty of violating the alcoholic beverage code if he or she negligently "sells an alcoholic beverage to an habitual drunkard or an intoxicated or insane person".
The misdemeanour carries up to a year in jail or fine of up to $500, or both.
Hight's blood alcohol level was four times the state's legal limit when he drove to his estranged wife's house, according NBC Dallas Fort Worth.
“Hight committed an American nightmare," Ms Glass's lawyer Scott Palmer said during an interview on Monday. "He was hell-bent on committing this heinous act."
But to blame a bartender two years later, he said, is not justice, but a stretch to hold someone accountable.
According to ABC affiliate WFAA, a lawsuit filed by the victims' families alleged Ms Glass told a colleague, Justin Banks, that Hight was "drunk and being weird" and brandished a gun inside the bar.
When Mr Banks escorted Hight to the parking lot, court documents said Mr Banks asked him to leave his weapons behind and him drive Hight home because he was not sober.
The lawsuit continued: "Hight told Banks he was having problems with his estranged wife and had something to do 'tonight'.
“Banks told Hight he should do them when he is sober to which Hight responded that he 'couldn't do the things he needs to do tonight without being this intoxicated'."
Although it is undisputed that Ms Glass served Hight alcohol, she took actions that were not legally required of her that night, too.
Court filings allege Ms Glass tried to convince Hight not to drive, called the owner of the bar to ask whether she should call the police (he said not to), and then she followed Hight to the house, although she left before the shooting began.
"There are allegations Hight foreshadowed what he would do and Lindsey Glass felt something was wrong," Mr Palmer told the Washington Post.
"Lindsey is the person who called 911. Not only did she know Spencer, but she was friends with Meredith and was supposed to be at the party that evening," he said, noting that the detectives commended Ms Glass for saving lives that day.
Lawyer Daniel Garrigan, who represents four of the eight shooting victims, said he was surprised it had taken this long for the police department to file a criminal case against Ms Glass.
"Clearly what she did was improper. Had she called the police or cut this person off from having more alcohol these people might well be alive," he told the Washington Post.
"When you've got a case where the acts of omissions of the bar resulted in eight deaths, I don't think holding them accountable is inappropriate."
However, according to legal experts, Mr Garrigan misstates the law.
Under the Texas statute, which Mr Palmer and Mr Garrigan both said is rarely applied, Ms Glass had an affirmative duty not to serve alcohol to someone who she should have known was intoxicated. What actions Hight took after that are not relevant.
Whether Hight murdered eight people or eight hundred in a drunken stupor should have no bearing on Ms Glass's case.
The issue is whether Ms Glass served an insane or intoxicated person.
"It comes down to whether a reasonable person in the same situation as Glass would have known Hight was intoxicated," says Kenneth Williams, a criminal law professor at University of Texas Law. The answer is a question of fact that ends with the passing of alcohol from the bartender to the bar patron.
There is always the danger of bad cases making bad law. This case certainly raises the concern, according to George Washington University Law School professor Jonathan Turley. And because the shooter was killed, there is a temptation to place responsibility on someone among the living.
"It's easy to take a horrific criminal act and associate it with an earlier crime," like over serving alcohol, he said.
"Glass may have had reason to know he was intoxicated; however, the charges seem to be an outgrowth of the homicide as opposed to the over serving of alcohol violation."
Ms Glass’ charges could be interpreted as an eleventh-hour effort by the Plano police department to hold someone accountable, or a sign that criminal law is trending in the direction of criminal affirmative duties.
It is important to understand where the line of criminality is set by society, Ms Turley said, because these cases blur them in a potentially dangerous way.
Some of the facts alleged that Ms Glass had reason to suspect Hight was going to do something criminal, though he never said it explicitly. Still, there was no legal requirement she call the police. No law required Ms Glass to reach out to the owner of the bar or to follow someone she thought would be a danger to others.
"As bartender did she have an affirmative duty to act or affirmative duty to intervene? – likely no, but like the old saying goes 'see something, say something' – that is precisely what Lindsey did," her lawyer, Mr Palmer, said in an email.
He called her decision to follow Hight to the party an "act of heroism" that saved lives.
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