Pure speculation, dont think I believe this myself but wonder what you think:
They set out actually with the intention of looking for work, not necessarily murder or crime at all, thought of themselves as badasses because of the media they chose, testosterone etc, thought the couple was an easy mark to make a few bucks on a dark night, something went awry and couple is killed. panic sets in and they flee, kill the man in carjacking gone wrong, continue to flee in a panic.
Variables on the above
-"looking for work" could have been robbery or some sort of crime from the get go, so they could be prepared and have weapons (obvs they do) but somehow things dont go as planned and their actions now are still motivated by panic on some level
-possibly they killed the man purposefully thinking him some sort of witness, or thinking already in too deep etc.
idk something just doesnt add up to me about them going on a killing spree or murder being its own motivation, or there would be more violence? Possible there are victims we dont know about yet but it seems theyve had alot of opportunities to harm more people, or put a better way they seem to be trying to hide not going on a rampage, idk just thinking aloud really