Ace came for him, Ace attacked him. He took the best out of the situation, defended himself. He is a pirate after all.
Whitebeard was already dying. Remember the big whole in his breast and his head? Blackbeard again just took the advantage out of the situation. Took what he wanted, before someone else jumps in.
Like Luffy literally did too already. Defeated big mums crew, took her territory (fishman island anyone?), or defeated blackbeards crew, took his territory (Dressrosa anyone?). Why is it worse in Blackbeards case?
Both were also on the move for Luffy already. At the end, they are marines and go against pirates. Happens against every other pirate too. Remember how Garp even let Ace die?
The big bad guy is Imu in my point of view. He is darkness, but this doesnt mean he needs to the bad guy, he is just another side of Luffys Medal.
You mean like... Jinbei, who was also in impel down, or like the devil child nico robin, or like a thief as called nami, or a pirate hunter... they are all pirates, criminals by default. We dont even know exactly what they have done to be put down in Impel down. Just speculation, but in the end someone could have been thrown there because he didnt saw some CD and walked against his slave. Not saying it happened that way. We just dont know.
Like every crew has these parallels, even since beginning. I even made the list for Kid pirates also.
My theory, he will, after Luffy got the One Piece and already won his place.