Tosfos is bothered by the seemingly strange Halachah that the Gemara derives from the Pasuk of "v'Ahavta l'Reiacha Kamocha." We are used to understanding that this means one should help others, and not doing things that one hates to others (see Shabbos 33a). Why does this Gemara say that this also means to make sure one dies a nice death? Where do we see that in the Pasuk? Tosfos gives two answers.

The first answer is that the Pasuk must be telling us something other than what it means, because a person is not even supposed to love his neighbor as himself. The Gemara in Bava Metzia (62a) tells us that ultimately "Chayecha Kodmim" -- "Your life is first," before that of your neighbor. Why did the Pasuk tell us "like yourself?" Though the simple explanation stands, this extra language must also be telling us something which is applicable about something other than life, namely death. This is why the Gemara has the ability to learn this message about killing people from the Pasuk "v'Ahavta l'Reiacha Kamocha."

The second explanation is that we see that the word "Nekamah" -- "revenge" can also refer to killing, as is apparent from the Gemara later (52b). Being that the prohibition of Nekamah is in the beginning of the Pasuk, there is room to learn a Halachah regarding killing from the second part of the Pasuk of "v'Ahavta l'Reiacha Kamocha." The Halachah of "Barur Lo Misah Yafah" is what Chazal derive from these Pesukim being close together.

All the best,

Yaakov Montrose