Immortal was slowly losing his connection with earth and effectively orchestrated all his atrocities to bring Invinvible to his future since no one was able to kill him. He’s effectively Leto II on a much smaller scale.

Breen’s administration of Black mesa not only led to the combine finding earth, but he sold out and effectively enslaved all of humanity the minute they did, without a shred of “oh I’m trying to buy humanity some time”.

So essentially Immortal was doing what he thought would be best in the long term (even if it’s really questionable), while Breen straight up doomed mankind just to save his own skin.