Well, you already know my suggestion for an Army Name. Hell's Angels.
And I know you said you don't want "Angels" in the name...but whatever.
For fluff, you could make your army the remains of various Biker units from septerate armies that banded together after a Nid attack on thier homeworlds. Since they were on Bikes, they could outrun the Nids whilist thier Marine brothers were ripped limb from limb by the wretched xenos. Now these remaining Bikers have sworn revenge against the Hive Mind.
How's that for fluff off the top of my head Blue
