As far as I can tell, Karna makes much more sense as an Archer than a Lancer. He only seems to be the latter for the sake of being the most suffering Lancer ever.
Many heroes can be summoned as multiple classes. Hercules from Fate/stay night could have been summoned as any class other than Caster, for instance. Karna's description indicates he can be summoned as Lancer, Rider or Archer, and it's possible whoever summoned him wanted him to be able to use his God-slaying lance, which he may not be able to use in other classes.
Does Joan of Arc's noble phantasm really kill her, or does it just weaken her considerably if she uses it right ? Cause it seems a bit silly that she can use it only once.
Yes, it kills her. It's the embodiment of her execution. She uses it, she dies.
Atalanta is described as Neutral Evil, yet her motives in the war seems to be Good, trying to create a world where children will be loved. So... which part makes her Evil here? This is getting complicated like how a sociopathic dick like Gilgamesh is referred as Chaotic Good.
Atalanta is most famous for challenging her suitors to a race and then killing them if they lost (which is kind of a dick move, considering she is the fastest human in Greek Myth) and generally being a pretty unpleasant douchebag, whereas Gil is famous for justly ruling the greatest empire on Earth for 125 by generally being more awesome than anyone else.
It's a moot point since David is no longer a servant, but since his sling allows him ownership over the target's weapons, what does that mean if he hits Gilgamesh? Does he gain ownership over every weapon in the Gate of Babylon? Does that mean that just Gilgamesh loses ownership of them, or does it also make their original wielders lose ownership of them as well? Or how about Fate/Stay Night Archer? Does it work against Lancelot's improvised weapons? It's clear that the writers didn't think his powers through. This is supposed to be his weakest Noble Phantasm.
Sounds like it. Unfortunately we can't see it in execution (I would guess that he can only steal items that are physically present—he could grab a spear that got thrown at him, but not open the Gate), but based on the Type-Moon wiki's translation, you're right.
The way it's worded on the Type-MOON wiki implies it only works on opponents with weapons at the moment of the sling's use. Arturia with Excalibur? Yoink. Gilgamesh with Gate of Babylon? No. The gate itself isn't the Noble Phantasm, the key used to open it is. As soon as he uses the key to open the gate, it disappears. Lancelot with Knight of Honor weapons? Sure, why not? Doesn't mean he can't make more though.