Tessa Farrell -- an actress who used to be engaged to the slain "General Hospital" star -- hopped online Monday with a heartbreaking reaction to the news that Johnny had been murdered in Los Angeles during an attempted theft of his catalytic converter.

The video is hard to watch, as Tessa gets emotional throughout as she responds to Johnny's death ... saying it's just a microcosm of what's been happening in L.A. lately crime-wise.

Tessa makes a call for action to revamp the laws here to make it so criminals aren't let back onto the streets so easily -- something that's been a hot-button issue for a few years now. Here, she makes no bones about it ... she says Johnny's murder was 100% preventable.

She also addresses his killer(s) directly ... saying that if they're watching, she hopes they know they got the wrong guy and that they shouldn't have resorted to violence over something as frivolous as a catalytic converter. She also says they should get a job.

TF has other fond memories and kind words to share about JW -- but her overall message is that his death is an outrage ... and she's asking for the public's help in bringing the people responsible for it to justice. She shared a link to a GoFundMe that his family started.

Now in terms of where things are in the criminal investigation ... our law enforcement sources tell TMZ that the suspects still haven't been apprehended, but we have more of a sense of what exactly led up to the murder. As it turns out, the shooting was completely unprovoked.

We're told Johnny was getting off a bartending shift, and we're told when he approached his car ... he saw three guys, who actually had the car lifted with a floor jack. Our sources say that when he got near them, one of the dudes fired a shot at him out of nowhere.

Like we said ... they took off and cops are on the hunt, and we're told police are canvassing the area for possible surveillance video that may have captured the incident.

Johnny was just 37.