After seeing what he can do with Poe(freezing him momentarily) and the blaster shot, couldn't he have just stopped Finn with telekinesis and taken Luke's lightsaber with his own hand?
Why even bother engaging him?
Ren had two motives for fighting with Finn;
He considers Finn a traitor and wants to punish him for betraying the First Order.
Finn challenged him to a duel. Ren is clearly enough of a ham that he can't resist.
Finn blocked him again and again, once letting the other man’s beam slide against his own and harmlessly off to one side. He counterattacked, to no avail. The longer the contest continued, the stronger Ren seemed to become. It was as if he was enjoying the challenge. Feeding upon it.
At least, it appeared so until Finn parried, swung, and unexpectedly stabbed, the tip of his lightsaber beam grazing Ren’s arm. That made it more than a challenge. Taking a step back, Ren reconsidered his opponent. When he closed the distance between them anew, it was with a purpose that had been previously lacking. Expecting an execution, he had found a contest. Now he had been touched. It was time for play to end.