Disclaimer: I Do Not Own Torchwood.
Set After CoE
He did something that the Doctor wouldn't ever agree with, but the Doctor wasn't there at his and Earth's darkest hour; he travelled all around the universe until he found the 456 home planet then wiped them out.
Jack would love to say that he felt the tiniest spark of guilt for wiping out an entire species, but his grief and rage made it impossible to feel it.
It didn't help him, killing them hadn't helped like he thought it would.
Because he still handed over those children to those monsters. Ianto and all the others trapped in that building were dead because of him. Alice, his little girl who he swore to always protect had lost everything because of what he had to do. Steven was dead because he made a choice to save millions of children at the sacrifice of his own grandchild.
Jack had rid the universe of 456 but killing the monsters didn't change anything that had happened or ease the hurt of so many.