Of course a .410 will kill someone...any gun will. But why would you want to rely on it when there are such better options? If that is your only gun and you cannot afford, or do not want to buy another...then load it up and be ready for defense. Whatever gun you use be sure it's loaded with good ammo and that you're proficient with it.
People who want to say humans are not hard to kill and compare killing them to killing a deer, here are my thoughts:
Humans may not be that hard to kill in a normal state of mind...what about the drug crazed lunatic that is crashing through the door? I want to biggest rounds possible and as many of them as I can have to stop that threat. I do not want him to die from bleeding out slowly or from shock later at the hospital, aftter he has killed/wounded me or my family becaue my gun failed to put him down right away...I want him dead as quickly as possible to end the threat. You compare shooting the deer with a .410 slug to a person...no doubt they will both die. But tell me this...how far have those deer ran after being shot? How many honestly fell dead on the spot unless it was a head shot? I will be willing to bet not many, if any of them at all. When I used to gun hunt I used a 30-06 with 150-180 grain loads and I still have had deer run after being hit. Now I primarily bowhunt and have seen it kill many deer quickly. So should I use my bow with a razor sharp braodhead for home defense? No, because even though it will kill...it will take some time. Think about it...if it takes that deer ( or bad guy in this case ) 30 seconds to a minute to die...that's a long time for him to hurt you or your family before the threat is gone.
"They're on our right, they're on our left, they're in front of us, they're behind us; they can't get away from us this time."
Chesty Puller, USMC, Chosin Reservoir, Korean War
"You cannot exaggerate about the Marines. They are convinced to the point of arrogance, that they are the most ferocious fighters on earth - and the amusing thing about it is that they are."
- Father Kevin Keaney, 1st MarDiv Chaplain, Korean War