When Kyoko overloaded her soul gem to kill Sayaka's witch form, does Kyoko then also become a witch? Or since she overloaded her soul gem, does she just die "normally?" If the latter is true, then why don't all magical girls just overload their gems before they become completely tainted, that way they don't become witches?
When Kyoko overloaded her soul gem to kill Sayaka's witch form, does Kyoko then also become a witch? Or since she overloaded her soul gem, does she just die "normally?"
Kyouko didn't become a witch, since she died in battle, rather than suffering a blackening of her Soul Gem.
If the latter is true, then why don't all magical girls just overload their gems before they become completely tainted, that way they don't become witches?
Well, if they knew that they would become witches upon having their gems completely tainted, I'm sure many would opt to make sure they die "naturally" rather than risk becoming witches. This is exactly the "solution" Mami opts for in episode 10's timeline 3, where, shortly after seeing Sayaka turn into a witch, she kills Kyouko by destroying her Soul Gem, and is about to do the same to Homura.
But there's the whole problem. Nobody (among the cast) knows this until they see Sayaka turn into a witch in timeline 3, and Homura doesn't tell anybody about this in the main timeline, so everybody else (including Sayaka) lacks this knowledge in the main timeline, too. I'm sure they figure they'll just die when their Soul Gems turn completely black (which is, in fact, what happens once the Law of Cycles is in effect), which means they have no reason to opt to kill themselves beforehand.
And, keep in mind what we know about how magical girls generally operate (from the show, but moreso from The Different Story) - magical girls usually work solo. The large number of magical girls working together that we see in the show is highly unusual. Consider the outcome: this means that nobody ever finds out that magical girls turn into witches when their Soul Gems blacken - if a magical girl, alone by herself, turns into a witch, who notices? Who's going to survive to tell the next generation of magical girls about the dangers of blackening their Soul Gems?
If they killed themselves, there would be nobody left to kill the witches. If they kill all the witches and then kill themselves that way, there could be a world with no witches. They would want to keep some alive to guard and tell new magical girls about that.
Not that it's relevant anyway, cause they weren't close to killing all witches, and the witches give birth to more witches.