My favorite (and "humane" way of killing these California spiny lobsters is to drown 'em in a bucket of fresh water.)
Two things happen:
1) As the lobster consumes the remaining oxygen from the water, it gently suffocates as it encounters hypoxia but since it is in its "natural" environment, it doesn't know that it is doomed. (This is very similar to how a human reacts when breathing 100% nitrogen, O2 levels deplete but the CO2 levels are "normal" so there is no 'alarming urge to breathe', it's just simply 'lights out'.)
2) Thanks to osmosis (the absorption of excess water into its cells, the tail of the "bloated" lobster expands and gently "pops out" to give easy access removing the tail and pre-shelling the lobster for culinary preparation on the BBQ grill.
After doing this, (10 minutes tops) be sure to prepare the lobster for cooking or storage as they can quickly spoil when they are dead.