The Gemara says [Sanhedrin 78a]:
אמר רבא ההורג את הטריפה פטור וטריפה שהרג בפני ב"ד חייב שלא בפני ב"ד פטור
Rava says: One who kills a tereifa is exempt as it is as though he killed a dead person. And as for a tereifa who kills another individual, if he killed him before the judges in court, he is liable to be executed. If the killing was not before the judges in court, he is exempt.
בפני ב"ד (מאי טעמא) חייב דכתי' (דברים יג, ו) ובערת הרע מקרבך שלא בפני ב"ד פטור דהויא לה עדות שאי אתה יכול להזימה וכל עדות שאי אתה יכול להזימה לא שמה עדות
The Gemara explains: In the case of a tereifa who killed before the court, what is the reason that he is liable? He is liable, as it is written: “And you shall eradicate the evil from your midst” (Deuteronomy 13:6), from which it is derived that there is a mitzva for the court to eradicate evil that it witnesses firsthand. In the case where the killing was not before the judges in court, he is exempt, as any testimony against the tereifa is testimony that you cannot render conspiratory testimony. Even if the witnesses testifying that the tereifa committed murder are found to be conspiring witnesses, they cannot be executed, as they conspired to kill a tereifa. And any testimony that you cannot render conspiratory testimony is not characterized as testimony, and is not accepted in court.
The Rambam says [Eidus 7-20]:
עדים שהעידו על איש טרפה שהרג והוזמו אין נהרגין שאפילו הרגוהו בידיהן אין נהרגין לפי שהוא טרפה.
When witnesses testify that a person who is treifa murdered a person and then the witnesses are disqualified through hazamah, the witnesses are not executed. The rationale is that even if they had killed him with their hands, they would not be executed, because he is treifa.
According to Rav Chaim, this Rambam is difficult to understand! It is true that killing the treifa with their hands would not make them חייב מיתה but the מחשבה to kill him is worse. Just as he said that if one conspired to kill a kotton he would be חייב even though theoretically for actually killing a kotton he would be פטור, so too we should say that for conspiring to kill a treifa he would be חייב even though for actually killing a treifa he is פטור. The מחשבה, asserted Rav Chaim, is more severe than the actual מעשה?!!
Now Rashi on the Gemara writes:
שלא בפני ב"ד - ואתה בא לחייבו על פי עדים פטור דאין עדותן עדות דהויא עדות שאי אתה יכול להזימה לקיים בה דין הזמה שאם הוזמו אין נהרגין דגברא קטילא בעו למיקטל:
The reason it is considered עדות שאי אתה יכול להזימה is NOT because if the עדים would actually kill him they would be פטור, so too if they conspired to kill him they are פטור as the Rambam says but rather because they tried to kill a "dead man", so they didn't do anything that would make themselves חייב מיתה. According to the Rashi, the יסוד of Rav Chaim can still hold true but according to the Rambam we have a problem!!
What is interesting is that the Rambam elsewhere seems to agree with the סברא of Rashi that the reason the עדים זוממין of a purported act of murder by a treifa aren't killed is because they did nothing of substance because they essentially killed a dead man [and not b/c of the comparison to killing him "manually"].
אדם טריפה שהרג את הנפש נהרג שנאמר ובערת הרע מקרבך. והוא שיהרוג בפני בית דין אבל בפני עדים פטור שמא יזומו ואם הוזמו אינן נהרגין שהרי לא זממו אלא להרוג טריפה וכל עדות שאינה ראוי להזמה אינה עדות בדיני נפשות: [רוצח ב' ט']
When a person who is a trefah kills another man, he should be killed, as reflected by Deuteronomy 19:19, which states: "And you shall destroy the evil from among your midst."
When does this apply? When he committed the murder in the presence of a court. If, however, he committed the murder in the presence of witnesses, he is not liable. The rationale is that the witnesses may be disqualified through hazamah. And if they are disqualified through hazamah, they will not be punished by execution, for they intended merely to have a person who was treןifa executed. And whenever the laws of hazamah cannot be applied to testimony, it is not considered to be valid testimony in capital cases.
In Nizkei Mamon 9-7 the Rambam is explicit that a triefa is halachically a dead man:
When an ox that is a t'refah or an ox belonging to a person who is t'refah kills a human, the ox is not executed. [This is derived from Exodus 21:29:] "And its owner shall also be put to death." [This is interpreted to mean] that a parallel is established between the owner and the ox being put to death. Since the owner is [already] considered as if he is dead and need not be put to death [by God], so too, the ox is not held liable.
[עפ"י הס' זאב יטרף]
