This week's parsha discusses a wide range of topics. Among them are the unique laws of eidim zomemim, the false, plotting witnesses who are given the exact punishment they planned to cause to the defendant. The manner in which they are refuted must be very specific as well as the timing of their disproof. The mishnah (Makkos 1:6) teaches that the refutation must come after the defendant had been sentenced but before the sentence was carried out in order for these laws to take effect. This is the famous conundrum of eidim zomemim, that the false witnesses are punished accordingly only if they ultimately fail but not if they succeed. The mishnah learns this law in cases of capital punishment from the pasuk (19:19) "And you shall do to him as he plotted to do to his brother." The fact that the Torah refers to the defendant as his brother indicates that he is still alive. If he has already been given the death penalty, the punishment referred to in this pasuk does not apply.
Similarly, the gemara (Sanhedrin 10a) learns from the pasuk (25:3) "And your brother shall be flogged before your eyes" that the punishment of lashes must not result in death for after the lashes, he must remain your brother.
Many commentaries question this understanding of the word ach, brother. After all, the pasuk (Vayikra 21:2) teaches us that a regular kohein may become tamei for the purpose of the burial of an immediate family member. There, the word ach is clearly referring to someone who is no longer alive. How can the aforementioned sources assume ach to be a living person?
Rasha"sh on the gemara in Sanhedrin offers an interpretation. The Torah uses the word ach in two different contexts. Sometimes it used to refer to an actual brother with a familial relationship. In this case, the relationship is not broken by death and thus, he remains a brother even after passing on. However, when the Torah uses the word ach to refer to a fellow Jew, the rationale is that he is your brother in mitzvos. He shares the same obligations as you. When he dies, he is absolved of his obligation to perform mitzvos and this brotherly relationship is severed. Therefore, the gemara justifiably extrapolates from the usage of ach that we are referring to someone who is still alive.