I know why Bard let them die!

Sure, he's a celestial being, but even they have limits.

Bard doesn't have the capacity to "kill an army" or fight off an entire force. I mean, just look at him. He naturally roams about. He uses magic, but his magics seem less on par with "I'ma blow shit up" like Xerath and more on the lines of "Let's start this ritual, so we can open this magically sealed vault."

If you forced him to fight, the relic would have been in Noxian hands. He would have been useless. And in the long term and big scheme of things, and if you go based off of logic as opposed to compassion, Relic > Individual lives.