Developing CityClan

In this analysis I'm generally referring to Scourge's underlings, not those who benefit from his rule.

Scourge. Leader of BloodClan.
Started out as an ordinary runt of the litter who was bullied by his two larger siblings. He's fascinated by the forest and has his mother teach him about it - he even ventures out into it, a sort of escape from his cruel brother and sister. It was everything he wanted it to be - magical.
The next time he visited the forest it was under much different circumstances. He was upset and scared, convinced that he no longer had a place at his home. So he went back to the one place he had been happy. This time, however, he had the snot beaten out of him by a cat who was a few months older than he was. Who he then vows to never forget. He knows he'll get back at this cat somehow in the future.
Soon after, young Scourge found himself in the city. Hungry, he tells a group of strays that he killed  a dog and took a tooth from it as a trophy. They don't believe him but they let him eat so he'll leave them alone - they don't have time for story-telling kittypets.
Word gets around. The strays tell some other cats that they met some little kittypet who said he killed a dog all by himself. Maybe they told it as a joke. Down the line, other cats tell it as fact.
Bone and Brick approach him and ask him if he can fight a dog for them. Brick and Bone are at least two cats who feel some vague sense of kinship with the other cats of the city since they cite their reason for wanting it gone as being so they and other cats can get to the source of food it is guarding.
Scourge decides that he would rather fight and be killed by this dog than leave - he is convinced that he has nowhere else in the whole world to go.
By sheer luck he's able to scare the dog away. Some cats of the city exalt Scourge as a hero and probably the same cats that believed he could fight dogs when they heard the rumor started to come to him for advice and leadership. Scourge gives them what he felt was sound advice and mostly told them to stand up for themselves (something Scourge doesn't have too much experience in.) His position as chief is left unchallenged, and the city cats have a sense of stability with Scourge around.
Then Brokenstar came to town, recently dispatched by Fireheart. And everything changed there. Scourge was reminded of his hatred for the forest cat who beat him up. Upon meeting the ShadowClan leader and his rogue warriors he finds that his position as a leader-ish figure of about two years is being challenged. Is this the first time? It may be.
Scourge kills one of Brokenstar's cats. Suddenly the cats of BloodClan realize that this leader of theirs is willing to do anything to protect their way of life, this sense of stability that he has brought to them. And they, in turn, are willing to fight alongside him. They encourage him. He now knows how he is going to exact his revenge against the cat from the forest.

Nearly two years pass after this. In this time Scourge's position as protector and advisor of the cats of the city has been changed by Scourge's renewed desire for revenge. He became a more active ruler. He didn't want anything before killing that ShadowClan rogue but now he has seen the bigger picture. Absolute victory over everything in his life that has ever tried to break him. He formed an inner circle of cats who would do anything for him - or, at least, for the power he offered, and he began to take everything he wanted by force. He has seen what a group of cats like Brokenstar's gang will try to do so he prevents all cats from helping one another, fearing being overthrown. He separates brother and sister, mother and kit, bullies his cats and even kills some of them, subjecting them to the same fear, loneliness and sense of helplessness that he went through before finding his place in the city.
At first, the cats that had relied on him as source of wisdom and protection, the same that rallied behind him when he killed that ShadowClan rogue, try to see the good in the new Scourge. They fear leaving him because where else will they find the same safety and stability? They want to believe that nothing has changed, that it's still worth supporting him.
Others change with Scourge and become the fighting force for BloodClan, filled with confidence for their wise leader, knowing that with him they will never have to worry about outside threats like the forest cats again.
Bone and Brick may fall into either category. To me, they seem to be the two cats who were the least threatened by Scourge.

The change happened. The city became a much dangerous place to live for all feral animals and housepets. It was especially dangerous for the cats living in BloodClan. I'm basically equating this entire situation to someone in an abusive relationship. The cats of BloodClan have nowhere else to go. They would rather stay, suffer and die than leave. Sound familiar?
No doubt there were a few attempts on Scourge's life. No doubt Scourge saw fit to punish many cats in response to this. No doubt Scourge ground these cats confidence into the dirt to ensure that this would never happen again.

At the end of nearly two years of a rule of terror, Tigerstar made his appearance. And so, with the new ShadowClan leader off to kill ThunderClan, Scourge was finally able to plan his absolute victory.
Can you imagine what it was like for these cats - these bullied, demoralized cats - to get the chance to kill all the forest cats? The cats everyone was so afraid of before? To get the chance to take revenge on someone, especially if these someones may actually be the source of Scourge's rage? Scourge must have made it sound so great.
Thinking about that, he may have taunted the weaker, fearful cats back in the city into fighting dogs and other cats so that they could feel powerful after Scourge let them know they meant no more than dirt, while the power-hungry cats fought for sport and praise.

Months passed and the day finally came. Tigerstar's death was gruesome - the swift execution of it, however, only reinforced BloodClan's fear of Scourge, and showed the forest Clans what they were dealing with.

Scourge gave the Clans his ultimatum. Then the day of the darkest hour came and it was a bloodbath.