She could in fact have shot first......and then had the gun jam (or maybe she was shaking too much, or something). She picked it up again later in desperation: this time it worked.
BBM - I always figured she may have tried to shoot him first but it jammed or she screwed up releasing the safety, but had her faithful knife in an ankle brace and as he tried to get out of the shower to stop her from trying to shoot him again, pulled it and plunged it into his chest. He likely did fall on her at that point(shock and slippery wet floor) but she kept stabbing/slashing at him as he tried to roll away and then used the sink to pull himself up to propel himself down the hallway to the exit as she kept attacking him. Once he collapsed from the loss of blood and probably oxygen, she slit his throat, dragged him back likely using one of the sheets to help leverage him, picked up her gun, released the safety or unjammed it and finished him off with her FU shot("still hate me brat!"), then started her cleanup. Anyway, I believe the actual shot was last simply because of the autopsy report.
http://murderpedia.org/female.A/images/arias-jodi/travis-alexander-autopsy-report.pdf
"The wound track perforates the anterior frontal skull near the superior orbital bone and
traverses the right anterior fossa, without gross evidence of significant intracranial
hemorrhage or apparent cerebral injury(although examination of brain tissue is somewhat
limited by the decomposed nature of the remains). The projectile re-enters the facial
skeleton near the midline and the wound track terminates in the left cheek."