动机

What were his motives in doing this? / What were the motives behind his actions? / What made him do it?

Revenge was the sole motive for the murder.

motive for committing a crime

chief/main incentive/motive; leading/dominant motive

real incentive; true motive

sordid/high/selfish motives

be well motivated; have a good intention; mean well

have impure/mixed motives

be politically motivated

conceal one's motive

see through/penetrate sb's motives

have a criminal intent/motive

be actuated by motives of ...; out of/through motives of ...

find a motive for