1. be the first to act against/accuse someone else
be the one to cast the first stone
This phrase is a biblical reference to the full phrase 'Let he who has sinned cast the first stone'; its meaning being that in order to accuse someone of behaving badly, you must first ask yourself if you have made any mistakes. The biblical quote is used to remind people that they are rarely in a position to judge anyone else's behaviour, so 'cast the first stone' means to be the first to accuse someone of questionable or wrong behaviour. Note that 'to cast a stone' can sometimes be used in place of 'to accuse': "I don't want to cast stones here, but there were a lot of problems that could have been avoided."