Sam Was Here (Nemesis) 2016 Movie Explained

Sam has not been able to reach his wife, Rita, or his child. This is the guilt he feels towards them.

Sam might have killed his family. It is shown by the woman making tea that he accidentally kills and how she tells him that he killed the little girl, which might be his own child. However, that is unclear.

Blood tainting the teddy bear is a metaphor of lost innocence. In a way, Sam had lost his innocence, which was symbolized here and proves the theory that he is the villain. 

The chained door is his prison cell where he can't leave or enter.

Nobody answers Sam because he was locked in a cell alone, which likely took him a while. (See Ouija: Origin of Evil (2016) Movie Explained)

Sam was in denial. He didn't believe that he was guilty. He wanted to scream to the world that he was guilty.

People in masks are his captors. He sees them in masks because they have no identity to him. Some of them might have also been his victims; hence he kills them quite easily. (See The Number 23 (2007) Movie Explained)

Some scenes show that Sam is very vengeful by taking unnecessary measures to kill his attackers even when he had already defeated them.

The ending symbolizes his death sentence being done. The lady cleaning the curtain off his blood shows that at the end.

Eddy is a presentation of the judge and the jury in Sam's head where his voice is different when he is the jury. He comes up with the sentence and doesn't get his hands dirty. He also provides evidence.

Rita, Sam's wife, said that he is dead and that man has been scaring her. She said that he has been dead for six years, which might be the time he spent in prison before his sentence.

The movie is a metaphor for the death sentence. There are a lot of indications of being in limbo such as the outstretching desert and the chained door, which is a sign of imprisonment.

The red light in the sky is likely hell awaiting Sam. He kept seeing it as it grew nearer and nearer.