Depending on how sympathetic you are towards Carl, his pleas to get Paul to come home in episode 8 are actually pretty sad.
To some degree, Carl knows the mask is all in his mind. When the mask does sound like Paul instead of the whole "your work isn't done yet" thing, it's mostly exaggeratedly yelling Carl's name as Paul once did... his memory isn't perfect, so his best efforts at keeping his only friend with him aren't quite right; his own mind just can't perfectly generate what Paul would say. And in Episode 12, after Carl finds that the real Paul is dead, the mask no longer floats or speaks and is worn around Carl's neck.
Before Carl kills himself, we get a chilling montage set to "Pachelbel's Canon" of all the death and decay Carl is responsible for. Every structure is crumbling, there are irregular cloud formations and there's not a single living thing to be seen. It shows just how sick Carl really is and how even he knows that it wasn'tworth it.
If that weren't devastating enough, before he jumps, his final words are him having a back and forth conversation with himself, pretending that he and Paul are having one of their typical quarrels and yelling "CAAAAAAAARL!" with tears streaming down his face before jumping to his death, at which point the music abruptly cuts out and we linger on a shot of his hat sinking into the water. Because that's what forgiveness sounds like: Screaming and then silence.
