This is the 6358th most frequent Chinese word.
手软
"Reluctant to act harshly or decisively; lenient or hesitant."
The term '手软' in this instance means 'merciful' or 'hesitating' during an act; here, it is used negatively to imply the person was merciless and decisive in their action.
他抢劫的时候毫不手软。
He showed no mercy while robbing.
The expression '手软' here means 'hesitating to act harshly,' and it conveys a sense of compassion or sympathy compelling a pause in one's actions.
看到那个小猫的眼神，她不禁手软了。
Seeing the kitten's eyes, she couldn't help but hesitate.
In this conversational usage, '手软' contrasts decisiveness; '不手软' is used to describe someone acting firmly and without hesitation or mercy.