The more I think about the gun, the less it bothers me that she used the knife first. Jodi put a lot of thought into this crime. FWIW, I'm one of the people who believes that she went there to give him one last chance to remember the good times. I think she apologized for whatever it was that Travis was angry about in his email to her, she promised she wouldn't do anything to embarrass him, had sex any way he wanted and then she asked for one last chance to make things work. He refused and she followed through with her plan.
Guns can be traced. She probably knew it would look suspicious that a .25 caliber weapon was taken from her grandparents and then the same size gun was used to kill Travis. Her first choice was to get him in a vulnerable position and then strike while blinded by a flash directly in his face. A gun that small would be easy to conceal in the waistband of her pants and she could get to it if the stab wound wasn't enough to incapacitate him. What she didn't count on was how messy it is when your victim is dying but still capable moving around. Stumbling to the sink and coughing up blood, crawling and leaving a trail, and possibly still capable of getting to a phone in the other room. So she stomped and stabbed him until was weak enough for her to deliver the fatal cut across the throat. That, however, made things worse. A huge amount of blood, he's still moving, and she has to get him back into the shower. In that horrific moment I think she panicked and used the gun as a last resort to finish him so that everything would stop.
Jodi is an evil person, but I don't think anyone can be prepared for that kind of carnage. It saddens me to think that line she attributed to Travis when she told the intruder story ("I can't feel my legs") was probably a detail from the actual crime. In the movies, the victim of a stabbing usually says something pithy (Et tu Brute?) and then collapses to the floor. Her victim didn't die quickly, and he was spreading evidence everywhere. The gunshot wasn't merciful, and it wasn't a symbolic coup de grace, it was just expedient.