It has to be proved that they are the guilty party.

尚需证明他们是有过错的一方。

《柯林斯英汉双解大词典》

We know he is the guilty party, because we saw him take the money.

我们知道他是有罪的一方，因为我们亲眼看到他拿钱的。

youdao

Whoever is the guilty party – the person who said it or the accuser – [should] get their due punishment

说话的那个或者是原告，无论是哪个当事人有错，他都应该获得他应有的处罚。

youdao

Glenavon's penalty was also for hand-ball, Andrew Kilmartin the guilty party this time from Sammy Stewart's cross.

BBC: Lisburn Distillery 2-2 Glenavon

Fortunately, with your help, this time around it will be much easier to identify the guilty party.

FORBES: Another use for security cameras: Keeping L.A. law firm toilets clean

Those who believe that guns kill people finger Apple as the guilty party.

FORBES: 5 'Google Maps for iOS' Arrows Splitting Apple Maps to Bits