Why would you bring an illegally obtained gun to buy a car unless you're planning to steal it? Correct me if I am wrong, I've never bought a car, but this does not seem like common practice. Why would you visit someone's house at night with your illegally obtained gun and then take them on a ride with you. Maybe, you're hoping that they live alone because then if you kill them there are no witnesses. However, if they do not live alone then you're plan is faulty because the people in the home can possibly identify you after. Thus, screwing up your plan. However, let's say you're determined to stick to your original plan to steal a truck and kill the person so there is no witness to your crime. You do this, but you're rattled because you know that there were people who saw you. Thus, you begin your clean-up right away and it is quite sloppy. The next day you see the man you killed has been reported missing and the lady you saw the night before is describing a tattoo that you have. You are officially "hot". You go about your business because you don't want to act suspiciously and attract attention. However, unless you involve more people you have absolutely no chance of cleaning everything up and acting normal at the same time. Maybe, you realize that you do not know as much about stripping down trucks as you think you did. So, you call your girlfriend and your partner-in-crime calls his and you explain the situation and for whatever reason they decide to help you. You give someone the gun, if you sell it you're making a profit and if you get away that's icing on the cake. You give your girlfriend the video because she has more time than you to deal with it. You have less time than you thought and the police catch you and now you are actually screwed. You text your girlfriend to make your friend to not testify because you no longer can.
I think if we try to think about this senseless crime logically, their actions make more sense.
Also, cheating isn't particularly relevant in terms of character assessment unless someone murders their partner.