I don’t think it’s a matter of either of them being capable of carrying it out on their own.
This took no special skill, and required only a vehicle and a weapon.
I think it’s a matter of this never happening if these two weren’t together in the first place.
This probably started as a fantasy, and by bouncing ideas off of one another they came up with a tentative plan.
So alone, the one who came up with this fantasy first might have just killed himself, or done nothing at all.
The other probably wouldn’t have considered it to begin with.
Together they were a toxic mix.