Picard's youth explains his reluctantance to destroy the Borg but was happy to watch other species die.

Picard's father and brother were, to put it politely, were dicks.

His brother regularly bullied him from a position of power as the older and favored son. Starfleet was Picard's chance to escape from that. So whenever the chance arises he decides to take out his pent up anger on people weaker than him by denying aid.

Like in Homeward where he watches as the planet's atmosphere. Or in Pen Pals (until Tin Man steps up and forces Picard to not watch people die).

However, when they rescue Hugh and develop the Borg killer thing, Picard wimps out because it would require the courage to stand up to someone who had the ability to hurt him.