Despite the ten commandments instructing Jews not to kill, and despite the instruction and an object lesson to not make rash vows, some 40 men made this vow:
Acts 23:14 We have bound ourselves with an oath to eat nothing until we have killed Paul.
They took this to the high council (the religious rulers), who joined their conspiracy.
Even if the men were misguided, the priests should have known better.
They must have seen Paul as a real threat, and perhaps it was because of the large number of Jewish believers and because of Paul’s previous standing in the power structure. There is no explanation of what the issue was, but it could have been power or money or keeping the peace with the (Roman) authorities, or keeping their religious and social status more generally, as these are the usual things that the powerful don’t like to lose.
Whatever it was, it led the high council to conspire to break the very Law they were working so hard to preserve.
The conspiracy failed. There would have been 40 very hungry men.