What's in a Name
The short (very short) History of Spike/Angel's fascination with name changes.
Our sweet boy was born Liam, which in Ireland is the same as our English William. The name means "protection".
After Liam was turned he took the name Angelus, the Latinate for Angel, after his sister Kathy mistook him for the spirit of her recently deceased brother.
While a vampire Angelus earned the name "Scourge of Europe" after terrorizing town after town in the company of Darla, Spike, and Drusilla.
Angel dropped the "us" from his name sometime after coming to America in 1905. It has never been stated in canon exactly when this change happened.
Spike was born William, which in England is the same as the Irish Liam. The name means "protector".
William was known, unfavorably, as William the Bloody in his lifetime because of his bloody-awful poetry. He kept the name after being turned, by torturing his victims with railroad spikes.
Shortly after being turned William took the name Spike. The most common theory that has come up in canon as to why is that a party goer before William was turned said he'd rather have a railroad spike through his head than listen to William's poetry, one presumes William obliged after being turned. He has been referred to as William by Buffy, Drusilla, and Angel after taking the name Spike.