Dayton James Webber — the quadruple amputee who is accused of shooting another man to death in his car — may have done so after an argument over a gun and a fight about a woman … TMZ has learned.

As you know … Webber was arrested for the murder of Bradrick Michael Wells last Sunday. Cops say eyewitnesses told them Webber shot Wells in Maryland, then asked them for help pushing his body out of the car. Officers say these eyewitnesses told them they refused, and they got out of Webber’s car before he drove off. He was later arrested in Virginia.

According to court docs obtained by ESPN, Webber and Wells got into a “heated argument” about “drugs, guns and a woman” before the shooting.

We spoke with Webber’s ex-girlfriend, Tori Mattingly, who said she was not shocked he is currently facing murder charges. She told us she dated both Webber and Wells … but there was no overlap between them.

Tori claimed she didn’t know what the argument was about… and it is unclear if she was the one at the center of their fight that day.

A family source tells us that Webber and Wells were hanging out earlier in the day at Budds Creek Motocross Park … and photos of them posing together were posted to Snapchat.

We’re told Webber and Wells were friends for about a decade … and Wells helped out Webber a great deal — washing his car, cleaning his house, and helping out at Webber’s family restaurant.