personally i dont think that his last words were anything profound. i like the idea of him and eric having one last moment together where they say something meaningful but i dont think it happened. the two of them were exhausted, disappointed, and eric was in severe pain from a broken nose. i think the last things they said to one another was probably several minutes before they killed themselves, and was probably some sort of question or affirmation, a “yeah” or an “ok” or a “let’s go”. then i think the two of them were quiet and introspective. they didn’t have to say anything else to one another. they knew everything already. you don’t shoot up a school with just anyone - they knew what the other felt, what they thought. they didn’t need to say anything deep. they just had to finish things.
as for dylan’s last thoughts, i think that it was probably a mix of a methodical list of things to do, mixed with disappointment, relief, and probably a bit of shock. he most likely saw eric kill himself, or at least heard it - this was confirmation that his best friend was dead and this really was the end. for all of dylan’s suicidal ideation, it still must have been scary to know that something as unknown as death was right around the corner. then he took the time to collect all of his personal belongings and pile them up beside him, and i think this was his way of preparing for the end. he stripped himself of all the things that meant a lot to him, one by one, and i think he did this methodically and calmly. it was something he had to do. i think he might have been a bit forlorn while doing this, maybe going over the disappointment of the bombs not going of, but i think this activity brought him from that to relief. once that was done, he was ready.
i like to think that by the time he raised the gun to his head he was at peace. while he didn’t die instantly, he would have been unconscious as soon as he shot himself, so i think his last thoughts were a kind of relieved “this is it” deal. dylan wasn’t happy while he was alive and had wanted to die for a long time. i like to think that his last thoughts were of relief that his suffering would be over.