The dialogues below are said by an old man to stop a fight between young guys in two separate speech balloons. The first one is a screaming-type speech balloon which he shouted at them and the later one is a normal speech balloon which he said in a milder tone.
First balloon: おっと よしな よしな くだらねえなぐりあいなんぞやるのは！
Second balloon: どうせ やるんなら堂々と正式の拳闘でケリをつけたらどうなんだい
According to my understanding, the particle 「のは」 used for nominalizing the preceding verb and making it to be a topic marker. I would like to know why it has to be put at the end of the sentence.