This

Don't get me wrong, ethically this situation becomes very complex, but let's not kid ourselves: the evidence is unfortunately very strong. In fact this is one of the easier cases for the prosecution to try imo.

To prove intent under the statute of Murder 2 in NY they have:

• a weapon him
• matching shell casings
• video evidence
• cellphone data linking him to a route away from the scene

Cause of death is self-evident.

There is no legal justification applicable here (self defense, ...)

There's also a chance this will get a terrorism enhancement. Supporting evidence would be the targeted nature, the casings and manifesto.

He can go for an insanity plea or extreme emotional disturbance, however this usually goes to the punishment phase and not the trial phase, making this almost irrelevant for the conviction.

The one thing he has going for him is jury nullification. This is legally dubious and usually can and should not be argued, for very legitimate reasons; however the jury can make their decision basically without justification and therefore this is very plausible. The probability is a different question which is hard to answer.

I do not like murder, but I sympathize a lot with him, however especially because of that I also have to keep it realistic and accept that legally the case against him is pretty strong.

TL;DR there's strong evidence against him, and if there's no jury nullification he will most likely be convicted.