Summing it up(Monolith!Cherry and World):
AU(or Au, whatever-) was 2 separate beings.
Monlith!Cherry and World(the other 4 personalities but as 1 beast). But World didn't have a murdering intention. The Past version of Monolith!Cherry(before becoming AU) didn't actually fuse with World until later. Until then, She would go around finding stuff and scavenging things to survive and stay away from the others, However, knowing Creator was the only nice one, didn't really bother their presence.
World at the time was a sloppy gooey mess, combining with anything or anyone just by touching it, so Monolith!Cherry had to stay away at ALL TIMES. World sometimes forgot about this and got to close at one point, nearly fusing the 2.
Summing it up(How Monolith!Cherry and World fused and made World extremely dangerous):
While Monolith!Cherry was out scavenging, caught the attention of the former most dangerous Monolith Beast, Corruption(Corruption cannot see, so is completely sound dependent) and chased her for a bit, until she managed to juke them and got back into her room, World carefully blocked the door while Monolith!Cherry sat to rest, but Corruption was already at the door, and kicked it open. Despite World's weight, Corruption's strength was able to send World flying, and... AU started to exist, and Creator just came to see the commotion, and... was terrified of what Corruption did.
World after fusing only knew violence, because as we know, Cherry has 3 sides to her, and each one went to the main 3. The first one being a bean, which went to the first one, making them less dangerous. 2nd being serious, which surprise surprise, went to the 2nd personality. And the final one, "I'm gonna f**king kill you" went to the 3rd, making them blood hungry and always wanting violence. Now the 4th however, wasn't affected by any of them. Having to pure of heart doesn't let you be affected by any of it.. And the 4th personality is the only thing keeping AU sane.
Caption this Meme