why does rashi say killing a bug is chayav a chatas. isnt it a melacha sh'ein tzrich legufa?
Simcha
Killing a bug is Eino Tzerichah l'Gufah, but only Rebbi Shimon rules Patur in such a case. Rebbi Yehudah rules Chayav. Here, this is Rebbi Eliezer's opinion, that [even] killing lice is like killing a large camel (even though lice are not "born" creatures) and goes with Rebbi Yehudah's opinion. See Shabbos 107b.
All the best,
Reuven Weiner