They have plans for Negan in future, It will be a stupid way to take rid of Negan after all the job ( sacrifices ) they did in the previous episodes :
- Killing carl and made him wrote those litters asking for peace
- Making Simon looks like evil and Negan the good guy.
- Making Rick killed the Saviors who escaped even he gave them his word ( to show us that this is not about evil and good + making rick reads Carl's letter ).
To kill Negan at the end in the way you suggested, all previous episodes must change.