Asgore wanted to rule over humans and monsters alike.
Asriel / Flowey wanted to kill all the humans and let monsters rule
Chara wanted to destroy everything
Frisk wanted to set the 6 other souls free and open the barrier for the monsters and humans to have harmony

Only one human soul was needed to open the barrier. One human and one monster soul. Monster souls however are unsustainable and fall apart if not strong enough. Asriel's however was strong enough for it to be taken from him, and used to open the power.
The seven human souls would allow any of them to break open the barrier and reach godlike power. And each had a different intention for what they would do after.

Long winded and probably doesn't make a huge amount of sense but there it is.

sry, but it's not true
- Asgore declared war by sheer hot-headed grief, he wanted to avenge his children's death, that's why Toriel left him
- Asriel just wanted to leave, he was just a victim of Chara, he represents the gamer of all time, who can not feel remorse, or affection, for he is able to load or save in case he ***** things up. As a flower, he was between living and dead, between monster and human, between player and NPC, he could not feel anything. Asriel only wanted to be himself again, but for that he needed ultimate control over the timespace so he could go back to when Chara and he lived.
- Chara was the only one who genuinely hated humanity, and wanted them to die no matter what.
- Frisk had no intentions, it's up to the player. Since Frisk and Chara shares the same soul I suppose Frisk was a reincarnation of the first child. It's up to the player to finish Chara's plan (genocide) or end the ******* .

Well, Chara hates life itself pretty much. He/she is a sadist of the highest caliber who enjoys making others suffer, and would most likely have destroyed the whole world.

Asgore, on the other hand, was going to use the souls in order to reclaim the surface and let monsters rule it in peace.

SPOILER!


so he can get out and murder the humans aswell. Asriel explains that Chara hated humanity, which is why he climbed Mt Ebott and fell down in the first place. Asriel also tell you how, when Chara died and Asriel absorbed his soul in order to bring his body back to Charas home village, Chara tried to take over Asriels body and slaughter all the humans, but Asriel managed to hold him back.

Its pretty safe to say that Chara is nuts by all accounts.

So... when Chara told Asriel about '6 more human souls' it was just a lie? He/she wanted to murder all humans? Or letting the monsters out so they could start the war over? (That could also mean the fact Asriel and Chara died, and Asgore declared war against humans, it was just Chara's plab B)

wait... what? Before Chara died he told the dreemurrs that he wanted to see the flowers of his home village again, so when he died Asriel absorbed his soul and crossed the barrier with his body. Asriel brought Charas body to the center of the village where chara tried to take over, but couldnt. the humans saw asriel with charas dead body in his arms and assumed Asriel had killed Chara, so they attacked him and mortally wounded him. Asriel managed to get back to the underground where he later died and Asgore, strucken with grief and anger as the humans had once again taken everything from him, decided to kill every human that falls down from now on in order to take their souls, cross the barrier and reclaim the surface.

Toriel became disgusted with his actions and left the castle to go live in the ruins.

An untold amount of time passes and 6 humans have died, and this is where the game starts.

In the True Lab if you check the VHS tapes, you can realise that Chara found out buttercups are poisonous, when they accidentally put it into the pie they made for Asgore. Therefore Chara made a plan to kill themself, then tell Asriel to take his/her soul and collect 6 more on the surface.
"Six, right? We just have to get six. And we'll do it all together, right?" as Asriel whispered to Chara on his/her deathbed

This is a nice summary. I, too, went back to talk with Asriel once again, also read tons about the lore to gain full understanding, but I still could not understand what Chara wanted to accomplish by breaking the barrier with 7 human souls, even sacrificing himself for that. But I might be wrong and his/her main plan really was to gain access to Asriel's body, and using the power of determination in a monster's body they could destroy humanity.