I was relatively new to MotoGP in 2015, having taken about a decade away from following road racing in order to recover from the politics and mid-season rule changes in Formula 1.
My impression at the time was that Rossi was old and crabby, and complained endlessly about people like Zarco when they raced him half as aggressively as he had raced his rivals.
Marquez had incredible speed, but he was recklesss and showed a lack of sportsmanship by frequently crashing into other riders to force his way through.
In short, I disliked them both!
I rewatched Phillip Island after "the incident", and saw nothing untoward. In Sepang I thought it was quite clear Marquez was harassing Rossi after the third pass. Things escalated from there, and fans of either rider seem to think the other got what they deserved.
I don't care for off the track drama, but it's easy to ignore. Escalating that to on-track drama with a hard pass or similar is unfortunate, but so it goes. Hard passes are part of racing.
The biggest failing I see is that in Sepang Marquez put the conflict with Rossi ahead of the racing. He was clearly faster. He should have put on a hard move, roughed up Rossi and kept moving.
At any rate, the event remains polarizing and a black mark on the sport. Both riders bear some responsibility, both have displayed poor sportsmanship at various points in their careers, both should have made better choices.