For those times when the ASG says you got N instances, but your cluster says you N - k instances.

terminator.sh

#!/bin/bash
set -e

CLUSTER=$1
aws ecs list-container-instances --cluster $CLUSTER | jq .containerInstanceArns[] | tr -d \" | xargs -I{} aws ecs describe-container-instances --cluster $CLUSTER --container-instance {} | jq .containerInstances[].ec2InstanceId | tr -d \" > cluster.txt

aws autoscaling describe-auto-scaling-groups --auto-scaling-group-names $CLUSTER | jq -r ".AutoScalingGroups | .[] | .Instances | .[] | .InstanceId" | tr -d \" > asg.txt

sort cluster.txt cluster.txt asg.txt | uniq -u | xargs -I{} aws ec2 terminate-instances --instance-ids {}