「まったく…こうなるとどいつもこいつも、殺{ころ}したほうが、世{よ}の中{なか}の為{ため}になり奴{やつ}ばかりに見{み}えてくる。」
「どいつもこいつも」 is a despicable and slovenly way of saying "everybody", so it is often translated as "every man jack", "fwits***", "all those bastards", etc.
「ばかり」 in this context means "all (of them)" as it is used in pair with 「どいつもこいつも」.
Regarding the 「なり」 part, as @snailplane pointed out, it would need to be 「なる」 for the sentence to make sense.
"Damn! Given the situation, (just about) every f***er is starting to look like one that would only serve the world better if killed."