Tranquilizers take time to work. More time than it takes a lion to kill the cubs. They’re lucky they were even there to film it. Such things generally don’t take place with human witnesses. I also don’t see the sense in risking human life to save cubs from what is simply part of life for lions. There is also a risk to the animals every time you intervene.
If you feel very strongly about this, go to your local park in the spring and watch what the ducks do to each other, then figure out if you can stop it from happening. Male ducks that don’t have a mate often try to force copulate with the females, and it can get very brutal. The females try to fly away or resist, and some of them are killed. It’s distressing to see, for sure, when you understand what’s happening (people often assume the ducks are ‘playing’ and don’t understand). But again, that’s nature.
Alternatively, stick to watching Disney movies where all the endings are happy. My former career was in animal behavior watching animals in the wild, and I do understand that it can be distressing to see the realities. I’ve seen footage of lion cubs being killed, so I know it’s hard to watch. But there simply isn’t anything you can do to change it on a practical level, and again, intervening can create more harm than good.