I am betting that GD cooked this plan up to rob mom's ex boyfriend. After all, having lived there he would know what items of value or interest DM had and where they were likely to be located on the property. I believe GD convinced JY that there were cool guns and knives they could take, maybe even money in a safe. I don't think they intended to kill DM. I think they meant to rob him and he unfortunately interrupted them doing just that. Dark clothing aside, DM would probably immediately recognized GD. At that point it became a free for all and they killed DM in a panic. again all just MOO
So GD thinks he will run away, I mean he can't go home and face the music that will eventually be playing. When he goes missing JY - freaked out about what happened at DM's and now very worried for his friend GD, confides in his father about the robbery/murder. His friend GD might be suicidal, worried there is no escape from the consequences of his actions. Meanwhile, GD has no means to run away, the guns he stole have been disposed of in effort to distance himself from the crime. He's already destroyed his phone to prevent being tracked back to the murder scene and on his great escape and finally abandoned his vehicle. So his big run away ends up being 36 hours of wandering aimlessly and laying low, all while still in the general local area of his crimes.
When found, GD concocts the ole biker buddy defense and his dad swallows it hook line and sinker since it beats believing his son is just an average thieving killer. JMO neither is the brightest bulb in the box.