Rava says that a mumar l'tei'avon which is someone who will only eat issur when there is no other kosher option is allowed to do shechita, just not allowed to check the knife (since he isn't believed to do things that require very much effort). The clear implication of Rava's statement is that a mumar l'hach'is, meaning one who ignores the kosher option and opts for the non-kosher food, his shechita is invalid even bidi'eved. However, it isn't clear why we invalidate his shechita. Rashi (d.h. chutz) writes - אבל להכעיס אפילו נמצאת סכינו יפה לא, דמועד לנבל בכוונה בידים. Rashi understands that we are concerned that he will intentionally mess up in the shechita and we therefore treat his shechita as a neveila. According to Rashi he is not inherently invalid, we are just concerned for intentionally botching the shechita. It should follow that if someone was watching the mumar l'hach'is carefully and determines that he didn't violate any of the 5 halachos of shechita, it would be valid. However, Tosafos d.h. kasavar, writes that the same source that we use to exclude a goy from shechita, וזבחת, from which we learn you need to be a בר זביחה, we exclude a mumar l'hach'is. Tosafos understands that to qualify as a bar zevicha you need to be someone who believes in the importance of shechita, therefore a goy and a mumar l'hachis who completely disregard the process of shechita are passul. According to Tosafos this is an inherent p'sul and therefore a mashgiach standing over him wouldn't help, just like it wouldn't help for a goy. The Shach (2:16) understands that this is a d'oraysa exclusion, and the product is a neveila d'oraysa. The Rambam has a similar approach to Tosafos that we equate a mumar l'hach'is to a goy, but is somewhat different. According to the Rambam we learn from the pasuk of ואכלת מזבחו to exclude a goy, and include a mumar l'hachis who has status of a goy. According to the Rambam the pasuk which declares the shechita to be treif is in the context of a goy, but we equate a mumar l'hachis based on the rule that a mumar has status of a goy. Whereas according to Tosafos, the pasuk doesn't speak about a goy, rather about one who is not a בר זביחה which incorporates both a goy and mumar equally. The difference between then is whether the equating of a mumar to a goy in this context relies on a general principal that a mumar is like a goy (rambam) or just happens to be that they are both excluded for the same reason which is not believing in the rules of shechita. - see page 15 of the pdf sefer on shechita that i linked to yesterday.