Steve went in search of the vampaneze, certain they'd accept him. He probably thought they were as twisted as he was. But he got it wrong. Although the vampaneze were killers, they weren't inherently evil. They didn't torture humans and they tried not to interfere with vampires. They went about their business quietly and calmly, keeping a lower-than-low profile.
I don't know this for sure, but I'm guessing the vampaneze rejected Steve, just like Mr Crepsley did. The vampaneze live by even stricter, more traditional rules than vampires. I can't see them accepting a human into their ranks if they knew he was going to turn out bad.
But Steve found a way in, thanks to that eternal agent of chaos – Desmond Tiny. Most just call him Mr Tiny, but if you shorten his first name and put it with his surname, you get Mr Destiny. He's the most powerful person in the world, immortal as far as anyone knows, a meddler of the highest order. He gave the vampaneze a present many centuries earlier, a coffin which filled with fire whenever a person lay within it, burning them to ash within seconds. But he said that one night someone would lie in the coffin and emerge unharmed. That person would be the Lord of the Vampaneze and had to be obeyed by every member of the clan. If they accepted this Lord, they'd gain more power than they'd ever imagined. Otherwise they'd be destroyed.
The promise of such power proved too much for Steve to ignore. He decided to take the test. He probably figured he had nothing to lose. He entered the coffin, the flames engulfed him, and a minute later he stepped out unburnt. Suddenly, everything changed. He had an army of vampaneze at his command, willing to give their lives for him and do anything he asked. He no longer had to settle for killing Mr Crepsley – he could wipe out the entire vampire clan!