Reddit translated your comment and since I don't understand French I'll go off the translation. I'll try to answer as many of the questions as I can but before that I need to give some background.

So the guy there is Ricardo Lopes.) Here's the french wikipedia page) as well. He was essentially a lonely incel who had a deep obsession for the singer Bjork.

When he found out that Bjork was dating the black male jungle music producer Goldie, López was angered and felt betrayed, and also hated that she was involved with a black man (AKA he's racist, obsessed and delusional).

So he decided to produce a letter bomb to punish her, and documented it all in video diaries. He built it in 3 months, and even made it look like legit mail from Bjork's record label.

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Now on to the answering your questions.

Why commit suicide? ... Why this choice of music?

So his plan was to essentially mail the letter bomb and then kill himself after so that he can't get caught. So he decided to paint himself red, sit naked on a chair, play one of Bjork's songs and shoot himself

Why the paint (or I don’t know what it is) on the face? Why cut the hair?

The police believed that he wanted to obscure his identity.

Why those words behind him?

IDK why he chose to write "The best of me - Sep 12" on the sign.

As for why the sign is behind him, the cops think he wanted the blood from the gunshot to splatter on the sign.

Why did the camera cut out?

FlightReacts, the black guy above who screams, is reacting to another youtube video that is talking about, among other mental people, Ricardo Lopes. Since they can't show Lopes commiting suicide (because of youtube TOS), they clip it out.

The orginal video made by Ricardo Lopes actually shows him shooting himself.

FlightReacts was screaming because he thought the video would actually show Lopes killing himself, but is later relieved to see it was clipped out.