Yes I've speculated on this as well. I even said a while ago that my husband was prescribed Adderall in his 20s (not exactly the same as Ritalin but similar effects) and had to stop taking it because he started having violent feelings and thoughts out of nowhere.
I feel like we could probably bet money on that drugs were involved. It just seems to make sense. First of all they usually are in these types of cases. Substance use can mean the difference between violent thoughts and violent acts, holding your tongue or starting something, joking about doing something really stupid like armed robbery and actually going and doing it, etc. I think when Bryer's friend told him "don't go out and kill somebody," and he said "I would never do that," he actually did mean it. Up until that point he was all talk when it came to violence. But drugs could be the missing piece that lowered their inhibitions just enough to push them over the edge into doing something terrible.
Also, two troubled oddball teenagers with a known history of substance use, out on their own for the first time, in the middle of nowhere where cops are unlikely to be around...are we really going to think that they stayed sober? Kam single-handedly driving halfway across the continent in three days...snorting Ritalin would certainly help with that. Their totally baffling and nonsensical decisions too. To paraphrase one of those "Meth: Not Even Once" ads, "Burning your car next to a murder scene to cover up the murder isn't normal...but on amphetamines it is." I'd bet El Chapo's entire fortune that they weren't sober.