Doing this to any animal—regardless of sentience—is straight fucked up. Nothing says selfish like torturing an animal to death so it tastes better. Make it quick.
I'm vegetarian, so I'm not defending this at all, but as I understand it, boiling kind of is the best way to kill a lobster quickly. A friend of mine had a roommate who thought otherwise and tried to decapitate one for dinner. I don't know their biology well, but there's something about lobsters' brains spanning the length of their body, so it wouldn't die. Just horribly suffering and bleeding everywhere with a knife through its body. Tossing it in boiling water should near instantly kill it. All the water inside their shell steams immediately.