Is there a difference between steaming and boiling a live lobster?

No, there is no difference in the process used. However, it isn't advisable to cook dead lobster unless you killed it immediately before putting it in the pot. Whole lobster spoil quickly which is why they are stored alive before cooking. As for whether it's humane to put a lobster in boiling water, their death is just a quick as any other form of death so I believe it's as humane as you will get short of letting the lobster live.