I'm almost positive Rossi caused the crash on purpose. He even looked at Marquez as they were coming out of the corner, you could almost see him thinking 'Allarighty then junior, mesa showa you whatta happens now.'
If this was automobile racing I might agree with you. The stability of the bike is tenuous as best and thinking that you might be able to "take out" your opponent by crossing a wheel without suffering a complete loss of control is illogical.
I do believe that he choose to take away Marquez's line, but only because of the expectation that Marquez was at least in control and cognizant enough to slow down and therefore give Rossi an even larger advantage than he had.
Years ago I was leading a race with one guy right on me the whole way. We had broken away from everyone. With 4 or 5 laps to go he started to show his nose on his best set of corners and kept coming up along the outside of me. I continued to run my line and give him that room on the outside because it was setting me up better for the next 3 corners and the finish line just after that. On the last lap as we came up to his best section I left the throttle open just a bit longer which took me to the very edge of the track, effectively taking away any drive he might have had. I won. In the pits I see the other guy marching toward me with his dad in tow no less. He was pretty hot and said, "You deliberately opened up your line and drove your bike to the edge of the track. If I had not chopped my throttle, I would have gone off into the gravel. I was on you that whole race. You never once took that line. That was where I was going to pass you for the win."
My reply was: "You got beat because you showed your hand too early, and yes, I did change my line to deny you the opportunity of a pass. And just so we are clear, I know I am called the "old man" out here, (I was 43 years old at the time), but make no mistake, I am here to win. Period. I will give you nothing. If you want something, you will have to take it, and then keep it.
The other guy just stood there slack jawed with his dad standing next to him grinning ear to ear. Dad turned to the kid and said: "Son, you just got schooled by a guy twice your age with 10 times your experience, now shake his hand and congratulate him."
Sorry, I raced a thousand races, and every once in awhile something reminds me of one of them and I can't help but reminisce a bit. Thanks for your indulgence.