Why did Adam Lanza truly wanted to kill himself?

If we want to find the motive behind the massacre we tend to concentrate on the question why Adam killed the children. But the question why he killed himself could even be more helpful to find out his true motive. First thing I thought was: obviously he wanted to die because he was suffering (no friends, no job, sickness...). But Adam himself says: „… don’t let anyone tell you that I want to die because life is so miserable, because that’s not my position … Life is a great thing and that’s the problem …“ What was Adams position? If he didn't want to kill himself because he suffered, then what was his reason? Any ideas or thoughts?

he hated culture. if i am remembering correct, he said something along the lines of people shouldnt be shamed for commiting or wanting to commit suicide, because suicide completely frees you from culture. if that is what he said, then it would not be a bad guess to assume wanting to be freed from culture at least played a part in it.

Likely due to severe anxiety, depression and sensory issues due to his autism, ocd and anxiety.

Sort of an elaboration to what marsh said.

Adam viewed human civilization as something which can only exist as a result of culture. Culture consists of value, if there was no value placed in any of the societal abstractions humans bind themselves to, human society could not exist (at least, not in the state it's in currently). Compare humans to other great apes (Adam often referenced chimpanzee society), in which they have very underdeveloped cultural memes that they pass down through their lineage, while humans have an immense amount of completely arbitrary values to abide by in order to conform to society. Adam saw this ceaseless imposition of arbitrary value in our civilization and he experienced it firsthand, just like everybody else, which led him to come to the conclusion that culture (and therefore life, since cultural values are endemic to human civilization) is harmful. For a cynic, it might be noted that despite denouncing value, Adam's denouncement is, in and of itself, valued! This is where suicide enters the equation: Since Adam recognized that he was just as deluded as any other human being who had been scarred by culture, he contrived that the only plausible way to escape this paradox of human suffering was to die. To Adam, death was neutrality and neutrality was bliss because "bliss" (pleasure/happiness/whatever) is just a momentary departure of suffering (instilled by the imposition of value).

I apologize if this is too long-winded or verbose. If you are really curious about his whole sort of "ideology" (if you could call it that), listen to CulturalPhilistine. If you don't feel like investing hours of your time into listening to him ramble, I recommend (in the case of your particular question) that you just listen to his 'My Antinatalism', as it's fairly short and addresses your question exactly.