I guess they wanted a final fight with Zenos and that they were building him up at the tail-end of SB and hints through ShB.
I feel like the team wanted to wrap up the OG storyline and Zenos as a result was a part of that.
But honestly, he's not very complicated to understand. Simply put, he just lives for the thrill of fighting and the WoL was that contender.
Personally I find it boring and it's kind of a weird addition to EW's ending but loose ends are tied up.
And for Zenos's sake, I hope he stays dead. Bringing him back would be pointless and make people dislike him even further than they already do.
The writers having him help us at the end was their attempt of making him into a neutral enemy.
And honestly him dying in SB is where he should have stayed, at least his death was poetic there.