When you register, the IP that you used when you submitted the registration form is used to calculate your evilness. The more frequently this IP or its neighbors were banned, the more evil is associated with your account. The amount of evil associated with an IP decays slowly over time, but the amount of evil associated with an account does not. You must pay or be manually whitelisted to enable posting on one of these "banned" accounts.

Here are some stats:

Evil	% new users
0	53
0-1	35
1-10	4.4
10-20	0.80
20-50	2.2
50-100	1.3
100+	2.9

Currently each unit of evil requires a payment of 4023 satoshi. You only need to pay something if you have 1 or more, though.