Watch the Bruins/Canes game before the deadline and you'll see why Canes fans are upset with Rantanen.
Him not signing is whatever. He didn't ask to be traded to Carolina, didn't want to be there, so him not signing was the expected result.
What WASN'T expected was how little he actually tried while in a Canes uniform. Regardless of how he ended up there and regardless of whether he was leaving by the deadline, for those 13/14 games, he WAS a member of that team and his play should have reflected that. It didn't. He played like he had one foot out the door from the get go, and it became especially obvious in the final 3-4 games.
So it's great that he's putting up empty net points in Dallas now, and I'm sure he'll continue to produce for you, but don't judge Canes fans for their reaction unless you actually saw him play for Carolina.