So. We all know Barry essentially (in a roundabout way) got Len killed because without his constant “there’s good in you” needling, Len would never have accepted Rip’s pitch to go time traveling with him and wouldn’t have sacrificed himself at the Vanishing Point.
But I just had a (terrible) thought: The original plan was to attack Savitar and get him trapped in the Speed Force, right? What made Barry change course and try to get through to Savitar and bring him into the fold was Len’s advice that the Flash should remain a hero and shouldn’t let the darkness in him guide his actions.
Except Barry’s plan went terribly haywire and resulted in a situation where Iris had to shoot Savitar, effectively leaving the Speed Force prison empty and leading to Barry having to take that place. And maybe that still would have happened without Len playing the unlikely angel on Barry’s shoulder, but… chances are that if it wasn’t for Len’s advice, Barry wouldn’t be stuck in the Speed Force now. ¯\_(ツ)_/¯
I mean, clearly those two bring out the best in each other, but with terrible results.