After thinking it over I can see kinda why the killed Carl off. I personally believe that it was to show once again that no one is safe from the virus and that anyone can go at any second.
It may also have been done to finally and completely destroy Rick. Look at it this way, his final tie to the world before the walker outbreak has finally been lost and with it sparks a realisation that to stop anyone else from dying he'll have to kill Negan. This is where i believe it's going to stem even further from the graphic novel because as I am to believe instead of killing him; Negan is instead put into a cell and held captive. However with his son and the last part of him connected to a time before the apocalypse dead it will push Rick into realising that the only way out of this is for Negan to die or else others will keep dying.
And no I don't believe Carl should've been killed off at this time especially with his character arc building up I can see one of the reasons why it may have been done. Of course it simply could be because Chandler was getting too old and they wanted him off the show; which is a possibility to be directly honest.
I personally want their to be a reaction to his death. I want their to be a big effect and for the war to end one way or another without it dragging on for another 2 seasons because that's when people get bored and characters get killed off.
TL;DR Carl's death was to symbolise the death of Rick before the apocalypse and that planning the war and what they're gonna do to Negan for 2 seasons at a time is why people get bored and people getting bored is why main characters get killed off.