The movie revolving around a magic wishing stone (seriously) and Wonder Woman inadvertently uses it without knowing what it is to bring Steve back to life. Except it can't bring people back fully, it is only able to take Steve's soul and place it into the body of a preexisting dude (what happens to that dude's soul/personality? beats me, the movie doesn't give a single shit about him). It is firmly established that this body is not Steve's body, it hasn't been reformed by magic, and only Diana sees Steve's face on the dude's body—everyone else sees the original body. So the wish has effectively stolen a body and crammed Steve inside it, leaving the original dude in some sort of limbo. Or functionally dead.

So even though both Steve and Wonder Woman know Steve is inhabiting someone else's body and is stealing someone else's life, they still have a bunch of sex, play dress up, and basically take their sweet ass time to get around to fixing the problem (and by the time Steve and Wonder Woman meet up, Wonder Woman knows the main villain has the wishing stone and is using it for bad purposes. they still decide to take like 24 hours off to go fuck and reconnect).

Eventually they discover the wishing stone always has some drawback, explicitly compared to the monkey's paw story. In Wonder Woman's case, the bad side effect of bringing Steve back was not, as you might think, stealing someone's body and casting their soul out. It is, instead, that Wonder Woman very slowly loses her powers. So Steve eventually convinces Wonder Woman that she must give up her wish in order to get her full powers back so she can stop the villain from using his wish powers to destroy the world and shit, and she tearfully does so (although she takes an incredibly long time and comes off astonishingly selfishly). Also I want to make clear that at no point is the original dude whose body Steve's inhabiting is never mentioned. He is a complete non-entity in all this. It's fucked up!

At the very end of the movie Wonder Woman sees the original dude again, back in his own body, and they sorta kinda flirt and then the movie ends. So original dude didn't permanently die or anything, but his body was used against his will for several days: it was used as a means for Wonder Woman and Steve to fuck, and it was brought into serious danger multiple times as Wonder Woman and Steve try to thwart the villains. That's so fucking messed up!!!

And the best part of all this is there is literally no reason they had to do things this way. The magic wishing stone later on is perfectly capable of materializing things out of thin air: including nuclear missiles, a giant wall crossing a whole nation, etc. There's very little reason that Steve had to have been brought back in this very specific way.