When Hiro Nakamura was freed from Evernow on Heroes Reborn he brought the spirit of the original show with him. Bouncing back from the video game in fine form, he promised Noah Bennet that they would not be stepping on any butterflies when they traveled to the past. He had seen first hand how dangerous changing the past could be, and he wanted to tread lightly as they returned to Odessa, Texas. Together they went in search of Claire, and to stop the bomb that triggered a mass anti-Evo sentiment in their current timeline. Still an incredibly conscientious man Hiro was true to his word, and before taking any major action would time travel ahead, examining the consequences before acting. He and Noah fight Harris and his many clones, trying to determine where the bomb is. Freezing time, it looked for a moment like Hiro would kill Harris once and for all, but after a rapid succession of time travelling he simply disappears, leaving Harris alive.
So after fighting against Renatus to save humans and Evos alike Hiro chooses to not stop the Odessa bombing, and in the process allowed scores of people to die. For a show that's already killed off its main characters and flirted with the apocalypse, this was shocking to see — he could have saved so many people, including Luke's darling little boy (who sadly seems to have died for real in the explosion, no tricks here). We don't yet know what the world would have looked like if he hadn't stopped the bomb, but it must have been pretty grim for him to stand by and do nothing. Hiro has so far only said that there were "too many butterflies" involved, vague but true to form.
Personally I'm optimistic that by not stopping the explosion they are able to prevent the end of the world, and if the explosion had never been the world would have ended once and for all. High stakes, we'll have to wait and see where they go from here.
Images: Sophie Giraud/NBC