Well I wonder why would they do it.It seems as though the Pharisees are trying to scare Jesus off.
If they really wanted him to die outside of Jerusalem, they could've just taken him away after capturing him.
What kind of motive you think they had except trying to save him?
Is it not possible that they are some of Rabbi Gamliel students? The same Rabbi who saved the apostles had many students and was widely respected.
It's not such a far fetched idea to think that some Pharisees were genuinely worried for Jesus.