I learned of their ways(non spoiler)

Meaning he learned of the pathogen's power to bring the morph to life, an enchant creature that can come about from mixing with reproductive cells like those found in female embryos, plants etc...He learns this alien creature is feared by the Engineers and revered...he also learns why they have rejected mankind for its greed, weakness and evil....

maybe the engineer ways are to seed planets and when the civilisations grow they worship these elders.

what do the elders gain from this?

this is a struggle for me.  i think its more than religion or the odd sacrifice.

david finds something so shocking he deploys the black death on that city.  im not accepting its some grudge he has with man/engineers.

going back to prometheus.

2000 ish years ago engineers 223 tried to get to earth to "destroy mankind"? we think

what if some ships did take off and drop a payload on earth. just not enough. however with the help of fire (and some being maybe) we defeated it.

it goes mistold in history as the black death or a plague.

what if the elder engineers thought earth was gone. however after the prometheus expedition when the hologram was activated or the engineer somehow contacted the elders before his death.

this warned them that the mission was failed and now they are trying again to destroy earth and have a 10 year start on us, since the prometheus incident.

one trailer said "mankinds greatest step may be its last" does this mean mankind on earth and if so why?

also if the morph in some form had been to earth in a bygone time, maybe  rumour or evidence was kept secret and an underground operation sold the stories to weyland yutani and that is why they have knowledge of the morph. maybe even more than they let on? 

I have to say, i dont know how you drive an android, who has already knocked off his own crew, to get upset about what the engineers might have been up to, I think david gets 'hijacked' by something he finds on the juggernaut.in the engineers script, david says that the engineers harvest lifeforms and dispose of them like waste, that is appalling, and could be what triggers him.

Strange indeed, to think that he is at all acting on behalf of humanity, when he had no issue killing Holloway and was the root cause of Shaw's baby  Squidward.  There are conflicting actions that he is taking.

Per the Cinemacon footage:

“The gods have rejected mankind as cruel, weak and full of greed,” Fassbender says in the trailer.

“They are a dying species, grasping for resurrection. So they are leaving the earth forever."

He continues,

“But their power is an illusion. They don’t deserve to start again, and I’m not going to let them.”

I think him saying this clearly states his intent.

David no likey mankind.

The planet he's bombing is probably just a planet like earth with similar human types, i.e.: mankind. 

He was probably just getting started but crashed somehow.

David might be in-part referring to the 'gods'...

“They are a dying species, grasping for resurrection. So they are leaving the earth forever."

The 'gods' did indeed leave us high and dry, I'd say.

“But their power is an illusion. They don’t deserve to start again, and I’m not going to let them.”

I get the feeling that David is talking of the Gods that are the dying species and he is out to put a stop to their agenda, also.

Although he could be talking about the gods in that quote, I think he is more likely he is talking about humans.  They are going to a remote planet to terraform and colonize because their home planet has become uninhabitable due to natural causes or due to humans destroying it.  It's possible the gods are in an eerily similar position, but I think the odds are against that.  I could be wrong though.  

I'm not sure if David ever really did respect the Engineers.  I think if anything, he views them the same way he views humans; in that humans created him "because they could", and he might apply that to the Engineers creation of humans, if he still thinks the Engineers did indeed create humans.  

Or if humans and engineers are different products of one big chain of evolution, his feelings might naturally expand to include every species that evolved from that same chain.  

But I think it really comes down to... what did David learn while on the ship?  I think there are a few equally plausible possibilities.  

I got the sense they were going to paradise because of a distress call rather than to terraform it specifically. I do agree that the only way to interpret that quote that strikes me as reasonable seems to be to take it as an indictment of humanity. 

You're right.  They had an intended destination but stopped at paradise due to a distress call.  My mistake!

I hope the movie give answers but in the spirit of Prometheus, gives us more questions