Choke Hold by Christa Faust

Angel Dare went into Witness Protection to escape her past - not as a porn star, but as a killer who took down the sex slavery ring that destroyed her life. But sometimes the past won't stay buried. When a former co-star is gunned down, it's up to Angel to get his son, a hotheaded MMA fighter, safely through the Arizona desert, shady Mexican bordertowns, and the neon mirage of Las Vegas...