Neon Genesis Evangelion

@re_player92:  
I'll answer your last question for you, there is nobody (only the creator of EOE) on this planet who know what happen at the end. Fans believe they understand, but not really

@re_player92:  
I guess, I still don't really understand it myself.  

@re_player92 said:

" So I finished the series today, including End Of Evangelion, and I just have a few questions that need clarifying.   1. I'm an idiot and basically forgot what caused Second Impact, can someone sum up the events that caused it?  2. I know Gendo wanted to cause Third Impact but what was so different between his and Seele's Third Impact? Was is something to do with Gendo wanting to be with Yui? 3. Who killed Kaji? 4. Was there any significance for Asuka saying, "How disgusting..." at the end of EOE?  5. Who's soul was bonded to Unit 00? 6. Can someone explain the ending of the series to me? People have told me that the end of the series is what's going on in Shinji's mind and EOE is what's going on in reality but I'm confused??? "    

 
Its been a while, but I think I'll be able to accurately answer most of these questions. I'll try to distinguish what is explicitly factual and what is just speculation: 
  
1. The Second Impact was the result of experiments on the First Angel, Adam, going horribly wrong in the South Pole causing a catastrophic release of energy. Essentially they were conducting experiments on Adam in hopes of minimizing the damage sustained by future angels, but the experiments backfired.
 
2. Gendo had personal reasons for bringing about the Third Impact: he wanted to reunite with his wife, Yui, since her soul was stuck inside EVA-01. In order to accomplish that, he was going to use Rei, Lilith and Adam in a process that would have been drastically different than what Seele actually intended - Seele wanted straight-up Human Instrumentality (turning everyone into that pool of LCL as in the End of Evangelion, making all of humanity  "one") and Gendo's divergence would've hindered the full realization of Seele's intentions. (For instance, once Gendo reunited with Yui it is possible that he did wasn't actually going to continue Human Instrumentality. We don't really know exactly since he is unable to fully realize his own goals because Seele decides to take matters into their own hands by the end.) 
 
3. Kaji was likely killed by someone he knew from Seele. Despite being subordinate to them, Kaji wasn't necessarily doing them much good (due to his personal relationships and interactions with people at Nerv), so they wasted him with the rest of Nerv as by that point of time he practically served his purpose of being a sort of liaison to Nerv on behalf of Seele. 
 
4. I think this had to do with their whole rejection of instrumentality causing them to become aware of each other's actions and thoughts to a degree - if you remember from earlier in the film, a stressed Shinji was implied to have pleased himself over Asuka's comatose body. Asuka probably caught a glimpse of it or learned of it while they were both temporarily exposed to instrumentality which would have allowed them to know a lot of personal details about each other. Her being aware of him doing that stuff while she was in a coma would have justified her reaction and comment "How disgusting" at the end of the film actually being directed towards Shinji's previous actions and not only the situation they were in.  
 
5. Unfortunately it is never actually made explicit whether or not Unit-00 possessed a soul. There is speculation, however, that because of how Unit-00 goes berserk and attempts to attack Gendo and Rei earlier in the series, Unit-00 possibly contained the soul of Naoko Akagi, a  researcher who had feelings for Gendo but ended up getting used by him in his fixation on his dead wife.  
 
6. What people told you is half-accurate and half-speculation. The reason I say this is because the TV series ending is pretty ambiguous. 
 
One interpretation of everyone saying "Congratulations!" to Shinji was that by the end of the TV series he actually accepts instrumentality and becomes one with the rest of humanity. The breaking of the walls in the scene prior can be interpreted to be analogous with his AT Field, which being broken down would have implied that he accepted instrumentality outright.  
 
The thing is, The End of Evangelion shows him explicitly rejecting instrumentality (which is why he and Asuka are left at the very end while everyone else is reduced to that pool of LCL).  
 
So the explicit, End of Evangelion ending is essentially the whole world going through instrumentality and becoming one by being reduced to a pool of LCL except for Shinji and Asuka, who psychologically rejected it and are spared from joining everyone else in the LCL pool. The ambiguous part is whether the TV ending is an alternate ending in which they actually share the fate of the rest of humanity or is, as others have suggested to you, just the psychological perspective of them rejecting it.   
 
Hopefully my answers are insightful for you!