Such an illegal act should not be treated lightly.

这样的违法行为不可等闲视之。

《新英汉大辞典》

Law evasion is an illegal act.

法律规避是一种违法行为。

youdao

What you did is an illegal act.

偷东西是违法的。

youdao

Police Colonel Omidi insists that Iranian government authorities will intervene in political matters on the Internet if there has been an illegal act.

VOA: standard.2009.11.14

An illegal act of defiance for which he was found innocent, as often happens.

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"Malfeasance" suggests wrongful doing, as in an individual intentionally performing an illegal act.

FORBES: Magazine Article

The researchers never know the result of the dice rolls, so they cannot tell if any specific individual may have committed an illegal act.

BBC: Study finds some Welsh farmers killing badgers