The famous yet non-existent drasha of כאשר זמם - ולא כאשר עשה - Rashi quotes it both in chumash and on the gemara 2b (top). But, in the gemara we don't find this drasha out all. The gemara expresses the concept using the term - הרגו, אין נהרגין - if they succeeded in killing the ni'don, they aren't killed. The closest thing we have as a source for this din is that the mishna interprets the pasuk of כאשר זמם לעשות לאחיו implies that the person their "brother" is alive and well. Why does Rashi (and tosafos 2b) seem to cite their own drasha from the words כאשר זמם?
The Poras Yosef 5b points out the the requirement of the "brother" being alive, which the mishna seems to consider necessary, would imply that even if he wasn't killed because of the testimony, but even if he just died due to natural causes, the witnesses couldn't become eidim zomimin. The Ritva points out that this is not true. The Ritva writes that the real source that we can't create zomimin if the ni'don has already been killed is that the Torah describes כאשר זמם and we can't include in that more than it says (since we can't learn mi'sah from a kal v'chomer, as it says in the gemara). Perhaps it is for this reason that Rashi and Tosafos don't want to quote the pasuk of "a'chiv" which implies he is still alive, because it would then seem to include even cases where he just dies, therefore they quote כאשר זמם, ולא כאשר עשה. Rashi and Tosafos don't mean that this is a drasha, rather they are saying that since the pasuk only speak of what they tried to do, the only possibility of including what they actually did would be a kal v'chomer, but since we cannot learn this from a kal v'chomer, it doesn't qualify as a case of zomimin.
The Meshech Chochma offers a very nice explanation to justify the concept of ולא כאשר עשה. It is likely that if witnesses succeed in causing a killer to receive capital punishment, his family will hire false witnesses to turn the first group into zomimin. So long as they haven't succeeded in their act, the family would be smarter to hire false witnesses to simply contradict the first group, creating contradictory testimony, which would force the beis din to acquit the killer. This would be a safer system since there wouldn't be a chance of another group turning the second group into zomimin. Therefore, if there is a second group trying to turn the first into zomimin, we trust their testimony. But, after the killer has been executed, they want revenge and would have incentive to hire false witnesses - that is why we cannot believe the second group after the killer has been executed.