Even then there are degrees of premeditation. The narrative that appears to be emerging is that he was suffering from long-term depression but that it was no longer visible. However, recently he'd received some news about his sight that implied that he would have to leave piloting. That's potentially devastating news, difficult to cope with at the best of times.
Feeling depressed he was suddenly presented with an opportunity to kill himself by crashing the plane he was in. Quite an easy one, in a way. No-one to stop you, once the door is closed. And at the same time you can feel somewhat detached from the process. Plane heads down, crashes, explodes, dead. Over quickly, gravity is doing the work, and maybe not much time between seeing the mountain and crashing into it. The passengers are, in effect, incidental and a tragic consequence -- it has not been established that he was deliberately trying to kill them and take as many with him as possible. The talk from the past about "leaving a mark", in this narrative, could be something of a red herring making this look planned when really that is more to do with mood swings.
Speculation, perhaps -- but a comparison with 9/11 is equally speculative, and also rather dangerous. It attaches a social stigma to mental illness that is extremely unhelpful. If anyone with mental illness is treated as equivalent to a potential terrorist then what message does that send out to the depressed? Don't talk about it else you will be locked up to protect the rest of society? Almost certainly this is not what is meant, and the idea of potentially suicidal people in charge of planes is one we obviously want to combat -- but the way to do that is to try and work on treating the condition, providing better support, better safeguards alongside and less of the stigma that goes along with it. Not more. Not comparisons with terrorists. Absolutely not.