Which one is right "Think bad of someone" or "Think badly of someone"?

In my opinion, "Think badly of someone" is right. But when I was watching a Vietnamese film with English subtitles "Think bad of me" was used.

Solution 1:

Certainly,

think badly of

is grammatical and idiomatic here. Macmillan has:

think badly of someone PHRASE

to have a bad opinion of someone or something

Nobody will think badly of you if you fail.

It considers this idiomatic because the adverb badly might usually be interpreted in the 'needs improvement' sense: He spoke badly.

think bad of

is also used {Google Ngrams}, though much less often; it is possibly modeled on the more clearly acceptable think ill of, which ODO wisely lists under 'phrases' rather than a particular POS section for 'ill':

Phrases

2 speak (or think) ill of

Say (or think) something critical about (someone).

Some would doubtless label 'think bad of' incorrect, judging it to be an unjustified flattening of 'badly' to 'bad'. But the usage note given at AHDEL needs careful consideration:

Usage Note: Bad is often used as an adverb in sentences such as His tooth ached so bad he could not sleep. This usage is common in informal speech but is widely regarded as unacceptable in formal writing. In our 2009 survey, 72 percent of the Usage Panel rejected the sentence just quoted.

In any case, 'bad' might be considered a noun in 'think bad of', as in 'desire good for'.