sci-fi movie from 2010+, probably a blockbuster - involving mutants [closed]

I have only watched Firefly and not Serenity, but the mutants you described definitely made me think of the reavers from the show.

By chance I found this question, which inadvertently provides an answer:
Why does Serenity have a cannon in “Serenity”?, Science Fiction & Fantasy Stack Exchange

The accepted answer there sounds like it's what you are looking for. Serenity was from 2005, but Firefly was known to be "way ahead of its time" so I would bet that the movie lives up to that.

You're looking for the movie Serenity.

It was probably a blockbuster

Alot of hoopla, but it was a flop. Budget $39 million Box office $38.9 million

I cannot remember the main story but it was around people flying in space with a mission

They were confronted to a race / mutated humans who were extremely aggressive and were flying from planet to planet to pillage and kill

on of the scenes (at the beginning I think) features the good guys (and one woman I think) closely escaping from them on a planet and they killed on their way out one poor guy caught by the mutants (to spare him torture and being eaten alive if I remember correctly)

They regularly mercy kill anyone taken by a reaver. Otherwise they'll rape us to death, eat our flesh, and sew our skins into their clothing – and if we're very, very lucky, they'll do it in that order. Mal does it right before a reaver ship chases them.

they also had to go through space between the spaceships of the mutants (who were having parties on the space ships) and "decorated" their own spaceship with corpses to look like the mutant's ones

To get through Reaver space, they have to look and sound like Reavers. This includes painting the ship with blood and tying up bodies to it.

I believe that the mutants were some form of experiment, if I recall correctly the scientist (a woman) who was working with (or rather - on) them left a video message explaining this just before being attacked and murdered.

The Reavers were an unknown side effect of the experiment to control human populations.

Another scene features one of the good guys telling a crowd to get into a bank safe and close it from the inside and not open - when the mutatnts were approaching.

From the script: MAL (to the guard, pointing to the VAULT) Does that open from the inside?

      GUARD
      Whah -ah- yes...

      MAL
      You get everyone upstairs in there and you seal it. Long as you got
      air you don't open up, you understand?

      GUARD
      I -- Buh I --

      Mal is in his face, dark and huge:
      GET THEM INSIDE THE VAULT.