The narrative makes no attempts to hide the fact that Sinister is an evil monster. He's not a saint. They are using him because he is useful and the knowledge Moira brings that the mutants NEVER come out on top lead to desperation and breaking all the rules. He is key to the resurrection protocols and he's an X-Man character, that's why he's here. it's already been established that he betrays Krakoa in one of Moira's other lives, so it's already setting up for the fact that he can't be trusted, and Moira may have already informed this to Charles and Magneto so they can plan for it.
In the meantime, people in the fandom love Sinister precisely because he's an awful person and loves every minute of it, but there is a difference between being entertained by a villain and thinking they're right in their actions