It's a thin line Batman walks there, but I could understand it. Bruce saved "Ducard" once and it bit him in the ass majorly. Saving him again would mean continuously endangering Gotham and the rest of the world from Ra's reprisals.
And besides... he might have survived. I'm not saying that cause I hold out hope he'll return in one of the sequels (like those insane Spidey fans who keep insisting Doc Oc is still alive), just that it's very much possible, however unlikely. Ra's is a master ninja, he might have made his escape in the last split second. It's not like Bruce stuck around to check... Bruce just refused to take Ra's fate in his hands, he left Ra's squarely to his own destiny. If that meant death, so be it...