If a rodef kills the person trying to kill him, is he chayav misa? Or is it similar to the case of the zealot where if he is killed in self-defense by his victim, the victim is not chayav?
If a rodef were to kill the person saving himself he would be liable. There is a big difference between the case of a Rodef and the case of the zealot.
In the case of the zealot although it is an Halacha L'Moshe M'Sinai we tell the zealot NOT to do it. Therefore we do not hold the philanderer guilty for saving his own life (Sanhedrin 82a and Rambam Issurey Biah 12:5)
In the case of the Rodef the person killing him is doing the absolute right thing. There are two explicit posukim saying to do so לֹא תַעֲמֹד עַל דַּם רֵעֶךָ and וְקַצֹּתָה אֶת כַּפָּהּ .If he doesn't want to get killed he should stop what he is doing. Otherwise because he wanted to kill someone and someone else tried to prevent it so he killed that person instead he should be left off?
The Achronim ask why Zimri wouldn't have been liable for killing Pinchos even though he could have saved himself by stopping to do the avaira. They answer that for him to stop he would have had to do so in the same way a person stops having relations with his wife if she realizes that she is a nidda which is not right away. Therefore they did not make stopping a condition to not being killed if he killed the zealot.
It's also not so simple that we let the philander off if he killed the zealot while the philanderer was also still in middle of doing the aveira. The Gemora above does seem to say he would not be help liable see the Maashe Rekach on the Rambam above for a discussion of the opinions about it.