There has been a great deal of discussion here about who had the right to do what, but not much attempt to understand exactly what happened. From the overhead video and the in-car videos, it is clear the critical moment was about 0.25 seconds before the collision. That was the point at which Masa turned right, placing the cars on a collision course (which they were not prior to that.) Why did he decide to do that? What was his thinking? I can think of only 3 possibilities.
1. He was unaware that Perez had departed from following the curvature of the edge of the track quite a ways back and had been travelling in a straight line for some time. He did not realize, therefore, that his right turn put them on a collision course.
2. He was playing a bit of "chicken"; he expected Perez would see him in his mirror and would turn right to avoid the collision (thus giving Masa a better line on the inside.)
3. He saw that Perez was taking an early apex and there would be no room for an inside move. He decided to cross close behind Perez (without lifting) and take the outside racing line, anticipating Perez (on the tight entry) would run wide and he would undercut him then. But he was caught out by Perez braking early (because he was nursing his brakes.)
Of these, 1 and 2 seem highly unlikely, so we are left with number 3. What else makes sense?
Can you define "for some time"?
If you make a pass on the inside on corner entry, you inevitably rely on the other driver seeing you, knowing that you're committed, and therefore knowing that he can't take your line because that would cause a collision. If the other driver wants to defend the inside line, he can, but there comes a point where it is simply too late to defend, and when you don't have much in the way of brakes left that point comes quite a long way before the corner. If the approach to the corner was straight and Perez had moved from the right-hand edge of the track, taken a diagonal line across the approach to the braking zone and the braking zone, I'm not sure Massa would have been criticised for not moving with him. It's only because the track, and the conventional line, goes right, that we have this discussion.
The line Perez was taking into that corner was a very slow one and contributed to the need for him to brake much earlier than normal, since in order to make the turn on that line he's have needed a rather slow apex speed. Of course, under normal circumstances he can take a weird line if he wants, but when there is somebody behind you who is closing on you quickly and looking up the inside, wandering across at 30 degrees to the normal line whilst braking heavily is not a clever way to defend your position, even if the other guy is still a bit behind you. Why did Massa turn right? Because that's the way the track goes and he was intending to make it round turn 1, is my guess. And at the speed he was closing on Perez he was expecting that, if Perez wanted to cover the inside, he would have done it much earlier.
Should Perez's initial failure to hug the inside of the kink have alerted Massa to the fact that he intended to drive diagonally all the way across the braking zone and take an abnormally tight line into turn 1? I don't necessarily think so. When it's too late for a driver to completely cover the inside line on the approach to a corner, it's not unusual to see them feigning to the middle of the track just to force the other car onto a slightly tighter line. But they usually return to the optimum line for braking. Accordingly, Massa probably expected Perez to realise the block wasn't on and go back to a more normal line.