Well, the blood angels were the first army I played when I jumped into 40k. And out of every possible race, army, whatever, the blood angels are my favorite all around. As for Zeros, I just like names that begin with a Z and that's the first thing that came to mind when I was registering.
"There is a cancer eating at the Imperium. With each decade it advances deeper, leaving drained, dead worlds in its wake. This horror, this abomination, has though and purpose which functions on an unimaginable, galactic scale and all we can do is try to stop the swarms of bio-engineered monsters it unleashes upon us by instinct. We have given the horror a name to salve our fears; we call it the Tyranid race, but if it is aware of us at all, it must know us only as Prey."