The problem here. Is we hear Kylo Ren speak in the trailer which is clearly not Luke’s voice. And we also see him without the mask. Also Kylo Ren is played by a different actor. Also Ren says in the trailer I will finish what you (Vader) started. Luke knows at the end Vader turned good and destroyed the emperor. Also look at Ren’s lightsaber. It said that that lightsaber is built by someone who doesn’t have the knowledge to build a proper one. That’s why there is exhaust ports. So it don’t blow up. Why would Luke build a unstable lightsaber when already knows how to build a proper one. Remember he built the green one in Return of the Jedi. So it would make more sense that he’d be able to build a red one without any trouble.