Why is it ironic that Okonkwo proposes to use violence to deal with the new converts to the white man's religion?

Doing so would violate the very customs Okonkwo is trying to preserve. is the right answer

The answer is that by doing so he would violate the very customs Okonkwo is trying to preserve, meaning that instead of being the bigger person he is becoming what he detests the most.

Another option would be that the use of violence would make him become the father he has always hated.