Blood evidence contradicts that theory - Dylan's blood was on top of Eric's pant leg when the bodies were found (meaning that Eric was already dead/sitting against the bookcase at that point in order for the blood to wind up there). This can be clearly seen in the suicide photo.
Also, Eric's brain matter was found underneath the chemical burn of the Molotov cocktail that was lit before Dylan committed suicide.
Randy's theory does not offer any explanation for this contradictory evidence. His theory is based on the function of the tech-9 and the fact that the magazine was found outside of the gun. I don't understand how guns work, so I can't say whether or not this part of his theory has any merit. I also believe he is biased against Eric due to Eric's history with his family, and he cannot understand the friendship between Dylan and Eric, and that Eric did have a good side and valued loyalty. For as much hatred as Eric had towards the whole world, he was also intensely loyal.
The only facts that might support Randy's theory are that, according to him, the tech-9 was found near Dylan's right hand, but Dylan was left handed. The entry wound was his left temple (I believe), and it'd be really hard to do that with your right hand. Others have said that, because the tech-9 was unsteady and the magazine stuck out, perhaps he held it with both hands (right hand supporting the magazine) when he shot himself. I can't clearly see from the suicide photos where the tech-9 is, so I'm not sure...