What is the word(s) used for someone who no one would suspect to do anything bad/evil?

Wolf in sheep's clothing

• someone or something that seems to be good but is actually not good at all

My grandfather was a wolf in sheep’s clothing – he looked like a sweet old man, but he was really mean.

(Cambridge)

innocuous

1. Having no adverse effect; harmless.

(thefreedictionary.com)

Depending on your sentence, innocuous looking might work better.

Edit:

Thanks to @AndyT: or seemingly innocuous.

Butter wouldn't melt in her mouth:

Be overly coy or demure; be insincere. For example, She looked quite innocent, as though butter wouldn't melt in her mouth, but we knew better. Already a proverb in John Heywood's collection of 1546, this metaphoric expression alleges that one is literally so cool that butter inside the mouth would not melt.

^{dictionary.com}

Used in my experience of someone who is actively attempting to appear innocent whilst not being so. Often used sarcastically where the attempted deception is transparent, but also as a warning where the deceit has apparently been successful.