So, it's not as cruel and inhumane as you think. If you're really, really squeamish about it, take the lobsters and put them in the freezer for ten or twenty minutes before you drop them in. They don't register the cold the way you or I do, and it renders them completely numb, so they definitely won't feel it then. It's an unnecessary step that panders more to your discomfort with the practice than anything else, though.
As for why we kill lobsters that way (by boiling them), we do so because the instant you kill a lobster, it literally spills its guts straight into its body cavity and starts to contaminate the meat. Either you cook it to death very quickly so the contents of its digestive system don't have time to ruin the flesh you're killing it to eat, or you kill it by dismembering it while it's still alive so none of the flesh is there to be spoiled by its digestive contents any more.
You can dispatch a lobster either way you prefer, but I would point out that this is a life-form designed to weather off having its limbs torn off and still go on living. Its brain will persist for much longer if you dismember it first than it will if you boil it and then cut it up afterward.
In any case, I don't bear any ill will toward my food, but nature is rarely kind enough to afford prey an instant death. I have to eat to survive. Something must die to feed me. Things suffer when they die.
The idea there's a "humane" way to kill something so it doesn't suffer is an illusion people labor under who haven't been forced to kill their own food, and I've had to hunt down and kill far too much of my own food over the years to believe it any more. No matter how fast we can kill them, it's never "fast enough". Not a single animal I eat escapes its measure of suffering when it's killed (whether I do the killing or an abattoir does), and I know it. That's the way it is.
I'm actually grateful to the animals I eat for what I take from them to ensure I don't have to share their fate, but in the end, no matter how grateful I am to them, I live with myself because I'd rather live with myself than die for the sake of my prey.
It's that simple.