While it ended up being a mistake, it was a good plan. Killing Rick would've caused chaos as without a leader everyone would do their own thing which Negan didn't want, he wanted them to work for him. So by killing some and striking fear into Rick, it keeps everyone in line.
Negan valued his people as no one ever! 0:07 Just as Rick's group would kill saviors all over again! They wouldn't think twice so Negan won't think twice!