Where we learn that Reginald Hargreeves actually caused the apocalypse by trying to prevent the apocalypse:
Step 1: Hargreeves commits suicide to bring his children together, hopefully to work together again.
Step 2: While back home, Klaus takes the opportunity to steal valuables and discards R. Hargreeves secret journal.
Step 3: Leonard/Harold, who has been stalking, dumpster dives to find the journal and learns about Vanya's powers.
Step 4: While grooming Vanya to destroy the Umbrella Academy, Vanya is triggered into becoming a murderous world-killer.
It is a bit of stretch that all the Harold and Vanya interaction happened in a week's time: The violin lesson, hooking up, falling in love, First Chair Violinist goes missing, Vanya auditions for the position, gets it, a trip to the country cabin, posters and newspaper ads are created with Vanya's picture for a concert that is happening in one or two days.
This is not the type of show that rewards paying close attention to details.